DIOPTOMETRY:
- CHAS. H. HELFRICH, M.D., SURGEON TO THE N.Y. OPHTHALMIC HOSPITAL.
By dioptometry is understood the methods for determining the refraction and accommodation of the eye. These methods are of two kinds-subjective and objective.
SUBJECTIVE DIOPTOMETRY embraces the methods which depend largely upon the statements of the patients themselves.
The method which is almost universally used, and which it is wise always to employ even though other methods are also followed, is that based upon the acuteness of vision.
It has been determined by experiment that the smallest distance separating two objects which permits of their being seen discrete is one that subtends a visual angle of one minute. Nearer than that they appear as one. The visual angle may be conceived to be formed by lines extending from the extremities of an object which meet at the nodal point of the eye, as in Fig. 20.
These lines represent secondary axes, which cross each other at the nodal point without undergoing refraction, and upon reaching the retina determine the size of the retinal image.
Snellen's test-types which are in general use are based upon this principle. Each letter as a whole, held at the distance marked above it, subtends an angle of 5', while the component strokes and the spaces between contiguous strokes subtend angles of I.
It is evident by the figure that the distance of the object is an important matter. The size of the object remaining the same, the angle becomes larger the nearer the object is brought to the eye; while conversely, the greater the distance the larger the object must be to preserve the same angle. Snellen's test- types are so designed that they are seen under a visual angle of five minutes when held at the distance at which they should be seen. The largest type should be seen at 60 metres by the normal eye, and from this they range down to a size visible at five metres. Fig. 22 shows them reduced in size. In testing the acuteness of vision, which is the first step to be taken, the patient should be seated with his back to the light and the test- type for distance placed opposite him at a distance of five metres or more, as space will permit. Such a distance is practically, infinity, and has the advantage that such rays which come from the card and enter the eye are parallel. Testing each eye separately, the patient is asked to read the smallest line of letters he can. His acuteness of vision (V) is expressed by a fraction, the numerator of which represents the distance of the test-card and the denominator, the distance at which the line of type he read should be distinguished. Thus, if he simply read the largest type at a distance of five metres his acuteness of vision would be expressed as follows.
V=5/60 It is important not to reduce the fraction, as it represents both the distance and the line read.
The abbreviations O.D. and O.S. respectively stand for the right and left eye, and are utilized for designating the eye examined. The abbreviation O.U. stands for both eyes used simultaneously. Should the patient's sight be so bad that he is unable to read the largest type, the greatest distance at which he can count the examiner's fingers should be ascertained. If even this is impossible, he should be placed in a dark room, and by alternately shading and uncovering a lighted candles his power to distinguish light should be noted.
After the acuteness of vision has been ascertained and recorded, the next step is the determination of the static refraction. In order to do this, it is necessary to possess a case of trial lenses and appurtenances, such as can be found at a first-class optician's, and several trial frames. The numbering of lenses is now almost universally after the metric system which takes as the unit a lens having a refractive power of 1 dioptre, and which has a focal length of 1 metre, or about 40 inches.
A lens of 2. D. is twice as strong and, therefore, has a focal distance of half a metre. Between the whole numbers are lenses of .25 D .50 D. and .75 D. The advantage of this system over the old or English system, where a strong lens was taken as the unit and where the number expressed the focal distance and not the refractive power,is that we are dealing with whole numbers in our calculations and not with vulgar fractions. It is a very simple matter both to find the focal distance of a given lens of the dioptric system and its equivalent in the English system.
If it be required to find the focal distance of a given lens of the dioptric system, divide 100 centimeters (1 metre) by the number of the lens and the answer will be the focal length in centimetres. For example, the focal length of 5 D. 100/5 = 20 cm. If the focal length is known and we desire to ascertain its dioptric number, we divide 100 cm. by the focal length, as for example with a focal length of 20 cm., thus 100/20 = 5. D.
In translating from the old inch system to the metric, we can consider to inches equal to one metre, and to obtain its dioptric equivalent, we divide 40 by the number of the lens in inches. For instance, No. 20 of the old system is equal to 2. D., for 40/20 = 2.
To convex lenses is given the plus sign (Plus), and to concave lenses the minus (-) sign.
In ascertaining the static refraction, each eye must be tested separately, as in the case of the acuteness of vision. Considerable advantage is obtained by commencing the test with convex spherical lenses, as these cannot be overcome by an effort of the accommodation. If these lenses increase the acuteness of vision, or do not make it worse, the refraction is hyperopic. Should the weakest convex lenses make the vision worse, concave spherical lenses should be employed. In the event of their failure to improve, convex cylindrical lenses are next utilized, and lastly concave cylinders. Even though the acuteness of vision is normal in the first place, it is still necessary to place convex lenses in front of the eye in order to determine if there is any manifest hyperopia present. Under such circumstances the strongest convex lens through which the said line of type can be read is the measure of the manifest hyperopia. In some instances the acuteness of vision may not be up to the normal, and no lens or combination of lenses makes it so, though the same line can be read equally well with convex lenses up to a certain strength. In this case the strongest lens also represents the manifest hyperopia. If convex lenses improve the vision to a certain degree, but short of the normal, recourse should next be had to convex cylindrical lenses in addition to the strongest sphericals found, which may bring it up to normal, the case being one of compound hyperopic astigmatism. The cylinder must be rotated in front of the spherical until the axis of the astigmatism is found. The strongest convex cylinder should be ascertained as in the case of convex sphericals.
In the event of failure with convex glasses, concave ones should be employed. Unless they actually improve the vision they are not to be considered, because all eyes, no matter what their refraction is, can overcome the weaker concave lenses by an effort of the accommodation. Presuming, however, that they do improve the vision, the weakest concave glass which produces the maximum acuteness obtainable is the measure of the myopia.
If the vision is improved somewhat by concave glasses, though not up to normal, concave cylinders should be tried in addition to the weakest spherical obtained in the first place, and the combination may bring the vision up to normal. This would indicate compound myopic astigmatism.
Failing with both convex and concave sphericals, convex cylinders should be employed to find if simple hyperopic astigmatism exists. The cylinder should be slowly rotated in the frames in order to find at what axis it seems best. This being found, stronger lenses are placed in the frames at this axis until the maximum improvement is obtained.
In the case of simple hyperopic astigmatism the strongest convex cylinder represent the measure of it.
Simple myopic astigmatism is tested in a similar manner, but here the weakest concave cylinder is the measure.
In testing as if for simple hyperopic astigmatism, a certain improvement may be obtained, but less than normal. Leaving the strongest convex cylinder so ascertained in position, concave cylinders are added in a position at right angles to the former until the maximum improvement is obtained.
Such a combination composed of the strongest convex cylinder and the weakest concave cylinder is the measure of the mixed astigmatism.
This, in brief, is the plan to be followed in the examination of any given case, and if closely adhered to will prevent much confusion and loss of time. There are other methods of determining the astigmatism which must also be employed either as soon as its presence is determined or as a check upon results obtained after the former methods. Thus the presence of astigmatism is frequently discovered by asking the patient to look at the clock-face test- type, made up of lines, in series of three, radiating from a center in various directions. Wallace's chart is one of the most convenient. If astigmatism is present, one set of lines will stand out clear and distinct, while the others, but especially those at right angles to the first, will be indistinct. These designate the principal meridians of the astigmatism. Or the stenopaic slit may be rotated in front of the eye until a point is found where the vision is most distinct, which will designate one of the principal meridians, and as the emeridians are always at right angles to one another the other meridian is determined at the same time. Convex and concave glasses are now placed in front of the slit and the degree of hyperopia or myopia, if either exists, ascertained. Next, rotating the slit to a position at right angles to the first the same procedure is again followed out. If convex lenses improve or do not make the vision worse in one meridian, and concave lenses fail to improve it in the other, the case is one of simple hyperopic astigmatism. If concave glasses improve the vision in one position, and convex glasses make it worse in the other, simple myopic astigmatism is present.
If convex glasses improve or do not make vision worse in both positions, it is a case of compound hyperopic astigmatism. The difference between the strongest convex glass in each position represents the astigmatism, and the weaker of the two, thus found, the hyperopia. Compound myopic astigmatism is determined in the same manner by the difference between the two weakest concave glasses.
If the case is mixed astigmatism, convex glasses will improve or will not make the vision worse in one position and concave glasses will improve the vision in the other.
Numerous combinations and variations of these methods are made by different surgeons, but the same principles hold good throughout.
After the astigmatism is determined by any of these methods, it is usual to place the correcting lenses in the frames and have the patient look at the clock face, when, if the astigmatism is properly corrected, the lines will all appear similar.
In all cases of astigmatism, or in any case where spasm of the accommodation is found or suspected, the test should also be made under the influence of a cycloplegic.
Cycloplegics.-By Cycloplegics are meant those drugs which produce temporary paralysis of the ciliary muscles, and therefore suspension of the accommodation. The importance of this in determining ametropia has been stated in the preceding chapter. In addition, however, complete physiological rest of the eyes is obtained which often removes congestive conditions of the retina and choroid, and later when glasses are prescribed they give more comfort than they would have done without the use of a cycloplegic. The drugs most commonly employed are the sulphates of atropine, hyoscyamine and duboisine and the hydrobromates of homatropine and scopolamine.
(1) Atropine is usually employed in a strength of four grains to the ounce. Ordinarily it paralyzes the accommodation in about two hours and the effects remain for a week. A drop of the solution should be dropped into the outer canthus three times during one day. In cases of marked spasm of the accommodation in young hyperopic subjects it can be continued for several days.
(2) Hyoscyamine and Duboisine are employed in the form of solutions made up of two grains to the ounce. Their action is much more rapid than atropine and their cycloplegic effect more transitory.
(3) Scopolamine may either be employed in a one per cent. solution, a single drop being instilled, or in a one-fifth per cent solution, one drop every fifteen minutes for an hour and a half. Cycloplegia occurs in about forty-five minutes and lasts from three to five days. Toxic symptoms sometimes develop, so considerable care should be exercised.
(4) Homatropine used in a three per cent. solution, one drop being instilled every fifteen minutes for an hour and a-half preceding the examination, can be employed when a very transitory effect on the ciliary muscle is desired. Its effect is increased by dropping a drop of a four per cent. solution of cocaine in the eye each time before instilling the homatropine. The cycloplegic effects of homatropine pass off in about fifty hours, and are in a degree neutralized by eserine. It is not safe to use strong cycloplegics in elderly people on account of the danger of precipitating an attack of glaucoma. Of course, they must never be employed if glaucoma is suspected or present. It is unnecessary to use them in people whose advanced age denotes that the accommodative power is very weak.
Patients soon become familiar with the letters on a test card, and children are apt to memorize them before being tested, so it is advisable to have several cards with different letters. Thus a new card should always be displayed for each eye, and if at any time there is any suspicion that the patient is drawing upon his memory a different one must be substituted. In order to avoid the necessity of walking across the room each time to make such a change, and especially in order to be able to make it without the patient's knowledge, I devised a changeable test- type, the plans of which were presented to Messrs. Clairmont & Co., of New York, who made the apparatus and perfected the motor for working it. This instrument was described in a paper read before the Homoeopathic Medical Society of the County of New York in 1893.
It is illustrated in Fig. 22.
The apparatus consists of an ornamented wooden case, upon which are mounted the ordinary Snellen test-types. The five lower lines only are capable of being changed, the changes being produced by the revolution of five quadrangular rollers permitting the exhibition of four series of letters. Motor power is furnished by an accordion-pleated rubber tuber, which, when expanded by the column of air communicated to it by the pressure of a bulb, elevates a weight to which is attached an arm. As the arm moves upward it carries a cog which locks with the wheel that revolves the five rollers. This wheel contains four slots, placed at intervals of ninety degrees, for the reception of two catches, an upper and a lower one, which limit the revolution of the wheel to a quarter of a circle. After each quarter revolution the weight carries the arm back to its former position, setting the apparatus for the next change. It is operated by a bulb at the side of the patient, which is connected with the motor by tubing.
An instrument called the Refractometer has been invented by H.L. De Zeng, whose purpose is the estimation of the total refractive error and particularly the whole amount of astigmatism present in all the dioptric media without the use of a cycloplegic.
This instrument shown in Fig. 23 is manufactured by the Cataract Optical Co., of Buffalo. While disclaiming that any mechanical device can wholly replace the ordinary test under a cycloplegic this instrument is certainly of great value where a cycloplegic is contra-indicated or refused, as well as in general routine work.
In brief this instrument consists of a nickel tube, in the head of which is placed a stationery concave lens of 20. D. It also contains an inner tube which is movable along its cylindrical axis by means of a rack and pinion adjustment, and which carries at its front end a convex achromatic objective. These lenses in combination at different distances give all the spherical foci from plus .12 D to plus 18. D, and from - .1 D to - 0. D inclusive. The convex spherical effects are recorded upon a revolving dial at the side and the concave effects upon the top of the inner tube, visible to the observer through an oval opening in the top of the outer tube.
Owing to the range of the negative scale being limited to-9. D, two auxillary caps accompany the instrument, one containing 10. D and the other-20. D, which, when placed over the eye piece raises the negative scale to either-19. D or-29. D respectively. The outer tube is further armed at its front end with a revolving head, composed of two revolving discs, containing blanks, a stenopaic slit, and eleven minus concave cylinders set at right angles with their radii. The resulting combinations possible give a range of cylindrical lenses from-.12 D to-8.75 D, which can be rotated to any given axis, the latter being indicated.
By reason of the instrument's optical construction, it has an amplification of two and one-third diameters, and in consequence of this the test-types furnished with it are reduced to three-sevenths of the size of Snellen's letters, so that the visual acuity may be reliably estimated with the instrument. The instrument must be properly adjusted for whatever range is desired, either 3,4,5 or 6 metres.
The best method employed in testing is what is known as the "fogging system," which consists in over-correcting a hyperopic eye with convex lenses or under-correcting a myopic one with concave lenses which are too weak. The effects of this is to render the lines and letters deeply blurred, which causes relaxation of the ciliary muscle and in consequence latent errors to become manifest. With the instrument properly adjusted, and the patient properly placed, the thumb-screw is turned until the test-letters are distinctly seen and the reading noted.
Fogging is next resorted to by producing artificial myopia, which encourages relaxation of the accommodation. The thumbscrew is now tuned slowly back and the patient requested to watch the astigmatic fan. If he states that one or more lines appear distinct before the others he is astigmatic, whereas if they appear equally distinct simultaneously the error is simple hyperopia or myopia. If astigmatism is present it is necessary to utilize the concave cylinders contained in the revolving discs to render the lines equally clear. This procedure should be repeated to verify the result or to make any necessary corrections.
The power of convergence is tested with an ophthalmo- dynamometer, that of Landolt's being the simplest and best. It consists of a metallic cylinder blackened on the outside, containing a vertical slit 0.3 mm. wide covered with ground glass. Beneath the cylinder is attached a tape measure graduated on one side in centimetres, and on the other in metre-angles. The vertical line of light, produced when the cylinder is placed over a lighted candle, is the object of fixation. Approaching the patient in the median line until the patient sees the line double the near point of convergence is found and the distance in centimetres with its equivalent in metre-angles recorded. The minimum of convergence is found by withdrawing the instrument from the patient; but as it is usually negative, it is determined by the strength of the strongest abducting prism which will not cause diplopia with the patient fixing the object at six metres. If the number of this prism is divided by seven, the quotient will approximately give in metre angles the deviation of each eye when the prism is placed before one. The amplitude of convergence is equivalent to the difference between the maximum and minimum of convergence.
Accommodation is tested by means of the reading types of Snellen or Jaeger. A sample is shown in Fig. 24.
It is necessary to test each eye separately, and finally both together. The nearest point the type can be read represents the punctum proximum. This subject has already been discussed in the preceding chapter.
Presbyopia is to be determined after the static refraction has been tested. With the distance glasses in the frame, the patient is asked to hold the reading type or a newspaper at the distance at which he desires to work or read. Convex glasses are now added to the distance glasses, until the best vision at this distance is obtained. The optical equivalent of the glasses before each eye is now computed, and the resultant glass is presumably his prescription for near. The approximate amount of presbyopia at different ages can be computed from the chart (Fig.10) given in the previous chapter. This amount must be added to the amount of hyperopia and deduced from myopia. This should not be considered as final, but used simply as a check on the result obtained by testing and to save unnecessary loss of time.
OBJECTIVE DIOPTOMETRY.- Objective dioptometry embraces the methods of determining the refraction independent of any statements by the patient. These methods are valuable in conjunction with the test made by the trial lenses and test letters, and especially so when the patient is illiterate or too young to read letters.
Frequently they are utilized to save time in arriving at an approximate estimate of the error, the accurate degree of which is fully determined in the ordinary manner. Not that they are inaccurate in themselves when applied by an expert, but because it is a safer plan to check results.
Estimation of Refraction by the Direct Method- A qualitative estimation can be made with the ophthalmoscope held at a little distance from the observed eye. When it is remembered that the rays issuing from a hyperopic eye are divergent and those from a myopic eye convergent, it is easy to understand how an observer can see an upright image of the retinal vessels in the former and an inverted aerial image in front of the latter. If the observer moves his head from side to side, the vessels seen in a hyperopic eye will move with the mirror, the image being upright, while they will move in the opposite direction if the eye be myopic. At this distance no vessels can be seen in an emmetropic eye, because the pencils of rays emanating from any two points upon the retina (each is made up of parallel rays) will diverge from each other so that no rays will enter the observer's eye. Close to the eye, however, an upright image of the fundus can be seen. The quantitative examination is conducted with the mirror held close to the observed eye, if possible as near 13 mm., the anterior focus of the eye and the proper situation for the glasses to be worn. If the examination is conducted at a greater distance proper allowance must be made. In following out this method, it is imperative that both the surgeon and the patient thoroughly relax their accommodation. This is easily accomplished for the patient, if no spasm exists, when the examination is made in a dark room, but the examiner can only attain it after much practice. It is rather doubtful if any expert can relax his accommodation absolutely unless he be so old that he practically has none. Still, many can do so thoroughly enough to obtain approximately correct tests.
The most desirable point in the eye-ground upon which to focus is either the edge of the disc or the medium-sized vessels between the disc and the macula, especially two vessels running at right angles to each other. The macula is unsuitable, because of the contraction of the pupil caused by throwing the light upon it and the annoying corneal reflections obtained.
If the observer has any error of refraction, it must either be corrected with glasses or allowance made for it in computing the final result. all these conditions being fulfilled the examination is commenced. If the patient's eye is emmetropic, the vessels will be seen distinctly, and convex lenses rotated back of the opening in the ophthalmoscope will blur them. If hyperopia is present the divergent rays issuing from the eye are rendered parallel by the rotation of convex lenses, and the strongest convex lens through which the vessels are seen distinctly is the measure of the error. The convergent rays from a myopic eye are rendered parallel by concave glasses, and the weakest concave lens through which the vessels are seen most distinctly is a measure of the myopia. The direct method is used to determine the height of retinal tumors by estimating the refraction at their summit, and the depth of a cupping of the papilla of estimating the refraction at its bottom.
If the examination is conducted at a distance not more than 2.5 cm. from an eye, a hyperopia of 1. D will represent a shortening of the axis equal to 0.3 mm. and a myopia of 1.D a lengthening of the axis to that same amount.
Astigmatism can also be estimated by this method. It is known to be present when the upper and lower margins of the disc and the horizontal vessels are well defined while the lateral margins and vertical vessels are blurred, or vice versa.
Its presence may also be suspected if the disc is elongated either horizontally, or vertically, the long axis corresponding to the meridian of greatest refraction.
In estimating the degree it is the best to fix tow vessels running at right angles to each other and whose direction conform most nearly to the principal meridians of the astigmatism. If the vessels in one meridian are seen distinctly, while in order to see the vessels in the opposite meridian a convex or concave lens is necessary, the case is respectively one of simple hyperopic or myopic astigmatism. If, in order to see distinctly the vessels in the two principal meridians, two convex or two concave lenses of different strength is required, the case is one of compound astigmatism, either hyperopic or myopic. The difference in strength between the two convex or two concave lenses represents the astigmatism, while the weaker lens represents the simple hyperopia or myopia. The axis of the cylinder to correct the astigmatism is placed in the direction of the vessel which was seen through the strongest of the two lenses.
It seems unnecessary to again state that in hyperopic astigmatism the strongest convex lenses represent the measure and in myopic astigmatism the weakest concave.
In mixed astigmatism the vessels in one meridian are rendered distinct by convex glasses and the vessels in the opposite meridian by concave glasses. Hyperopia exists in the meridian at right angles to that in which the vessels were made distinct by the convex glasses and myopia exists in the other. The axes of the cylinders to correct this would be the reverse of this, because they refract only those rays which enter at right angles to their axes.
Estimation of the Refraction by the Indirect Method.- This method is not generally resorted to, because of its difficulties. It is sometimes used in estimating very high degrees of myopia, but rarely in hyperopia. The rays coming from an emmetropic eye being parallel they are brought to a focus by the object glass at its principal focus, whereas the divergent rays from a hyperopic eye are brought to a focus farther from the object glass than its principal focus and the convergent rays from a myopic eye nearer than its principal focus. The degree of the ametropia is determined by measuring this distance. Schmidt-Rimpler's method is usually employed.
Schweigger uses the indirect method in comparison with the direct method to determine the presence of astigmatism. It has already been explained that the disc appears elongated in the direction of the meridian of greatest refraction when seen in the upright image, so it is only necessary to state that the reverse obtains in the inverted image.
Skiascopy (Retinoscopy, or the Shadow Test).- This is a method of determining the refraction by observing the direction in which the light appears to move across the pupil when the mirror is rotated in various directions. The test should be made in a dark room at a distance of one metre from the patient. It is convenient to place a tape on the wall extending from the position of the examined eye to a distance beyond that of the observer. This is graduated in centimetres, and at appropriate intervals the corresponding number of dioptres marked. Either a plane or concave mirror can be employed, but the preference is with the former. The light is covered with an opaque asbestos shade having an aperture 1 cm. in diameter. If the plane mirror is used the light should be near the surgeon, but if the concave mirror is used, behind the patient. The arrangement is shown in Fig. 25.
Skiascopy has been elaborated especially by Jackson, whose description is here followed and whose work is the ablest published.
By rotating the mirror the area of light it throws on the face is made to move in the direction the mirror is rotated. Those rays which enter the pupil from a small light area on the retina, which also moves when the mirror is rotated. This area moves with the light on the face when the plane mirror is used and in the opposite direction if the concave mirror is employed.
For the plane mirror this movement is shown in Fig. 26.
When the mirror occupies the position A A the rays from L which enter the eye are reflected as if they came from L, and after passing through the eye are condensed at a, on the lower part of the retina. At the same time the light falls on the lower part of
Skiascopy by Dr. Edward Jackson. the face. If now the mirror is rotated to B B the light which enters the eye is reflected as if it came from l, and is condensed toward b, on the upper part of the retina. Simultaneously the light which falls on the face moves upward also. The same movement of the light with the light on the face occurs in hyperopia and myopia as well as in emmetropia.
The movement of the light area on the retina caused by a concave mirror is shown in Fig. 27.
When the mirror occupies the position A A the rays which enter the eye come from the focus of the mirror at l and are condensed towards a, on the upper part of the retina, while the light falls on the lower part of the face. When the mirror is rotated to B B the rays which enter the eye come from l and are condensed towards b, on the lower part of the retina, while the light on the face moves upward. The same is true in hyperopia and myopia. The real movement of the light on the retina, as it would appear from behind, is always with the light on the face with the plane mirror and in the opposite direction with the concave mirror.
In our examination, however, we do not see it in that way, but we watch the apparent movement as seen through the pupil. When the plane mirror is used the apparent movement in the pupil and the real movement on the retina are the same when the observer sees an erect image, and in the opposite direction when he sees an inverted image.
The reverse of this obtains with the concave mirror.
The rays of light coming from a myopic eye are convergent and cross each other at its far point, and proceed divergently. The point B (Fig. 28), at which they cross and which corresponds to the far point, is known as the point of reversal. As has been explained under myopia, the distance of the far point from the eye represents the focal length of the glass required to correct it, and, therefore, if the point of reversal is known the amount of myopia is also known.
Retinoscopy is an accurate method of determining the point of reversal.
In the following description it is assumed that the plane mirror is used, though it will apply equally to the concave mirror if we reverse the movement in the pupil and change the lenses. If the mirror is held closer to the eye than its point of reversal, as at A (Fig.28 ), an erect image is seen, and the light in the pupil will seem to move with the light on the face. Beyond the point of reversal, as at C, an inverted image is seen, and the light will appear to move in an opposite direction to the light on the face.
At the point of reversal it is impossible to see which way the light moves, and the illumination is much more feeble. At a distance of one or two metres from the point of reversal the illumination is very bright, but as the distance increases it becomes more and more feeble. Without altering the rapidity of the movement of the mirror, the apparent movement of the light is more rapid as we approach the point of reversal.
While the test depends mainly on the direction of the movement of the light in the pupil, the degree of illumination and the rapidity of movement aid in arriving at a diagnosis.
Myopia is estimated by finding the nearest point that an inverted image is seen (C, Fig. 28 ) and the most distant point at which an upright image is seen, as at A. Midway between the two is the point of reversal, B, whose distance from the eye should be noted on the tape for that purpose, and the number of dioptres corresponding is the measure of the myopia. Thus if C is at 55 cm. and A at 45 cm., which corresponds to 2. D. giving a myopia of that amount. When the myopia is of a high degree, the point of reversal lies very close to the eye, and in this situation a slight error in marking the distance may mean an error of a dioptre or more in estimating the myopia; whereas if a similar error is made when the point of reversal is situated at a metre or more from the eye it is unimportant. Therefore, in high degrees of myopia, in order to check results, it is well to correct all but about one dioptre by placing a suitable concave lens in a frame before the eye and measure the remainder, which is to be added to the lens in the frame.
If the myopia is less than one dioptre, the point of reversal lies so far away from the eye that when near it one cannot see which way the light moves. In this case put a weak convex glass in the frame to increase the myopia, then determine the point of reversal, and deduct the convex glass from the amount of myopia found.
Hyperopia gives an upright image no matter how far we recede from the eye, because the rays leave it divergently. In order to obtain a point of reversal, it is necessary to convert it into an artificial myopia by putting a convex glass in the frame and then finding the point of reversal as in myopia. The amount of myopia is to be deducted from the convex lens, the hyperopia being represented by the remainder.
Thus, if a convex 5. D lens is placed in the frame, and the point of reversal is found at 1 metre (1. D), the hyperopia will be 4. D. That is 4. D out of the five were necessary to render the divergent rays parallel; and the other dioptre to bring them to a focus at 1 metre.
Emmetropia acts the same as hyperopia, but when a convex lens is added the myopia produced equals the strength of the lens, proving that the rays were parallel in the first place. In astigmatism the refraction of the principal meridians is obtained in the same way as in myopia or hyperopia. In order to determine the refraction of a certain meridian, it is necessary to rotate the mirror about an axis at right angles to it, which causes the light to traverse the length of the meridian. The direction of the meridian is revealed by the areas of light assuming a band-like shape as its point of reversal is approached. This is most marked in the higher degrees of astigmatism. Near the point of reversal, where the band-like appearance is most distinct, it is easy to cause the apparent movement from side to side, but more difficult in the direction of the length of the band. The latter, however, is to be watched, as it determines the point of reversal.
If the astigmatism is of low degree, this band-like appearance may not be perceptible; but when we have determined the point of reversal of one meridian it will be found that there is still distinct movement, either upright or inverted, in the direction at right angles to this. Supposing the case to be one of myopic astigmatism, either natural or artificial, and the surgeon starts at a point nearer the eye than either point of reversal and gradually withdraws from it, the following phenomena occur: At the start the movement will be with the light on the face in all directions. Withdrawing to the nearest point of reversal there will be no movement in the meridian whose reversal-point it is, but direct movement at right angles to it. Between the two points of reversal the former meridian gives an inverted movement and the latter direct. At the farthest reversal-point the direct movement for the meridian ceases and the other remains indirect. Beyond both points of reversal the movement is against the mirror in all directions. The degree of the myopia of both principal meridians, either natural or artificial, having been determined,the astigmatism present is represented by the difference between them. As a final test the cylinder correcting the astigmatism should be put in the frame together with the concave or convex lens which will remove the point of reversal to about 1 metre, and the movements watched again.
When using the concave mirror the position of the observer does not admit of much change, the distance being generally one metre. The movements of the light are the reverse of those just described. Skiascopy is the most valuable of all objective methods, and the student should practice it industriously upon an artificial eye made for the purpose before he can depend upon his results in actual practice.
Ophthalmometry.-The term indicates mensuration of the eye, but it is usually employed to mean the measurement of the radii of curvature of the cornea and the corneal astigmatism present in an eye with the ophthalmometer. The ophthalmometer in general use in this country is that of Javal and Schiotz and is shown in Fig. 29.
The principles upon which it is based are as follows: The surface of the cornea acts as a convex reflector, the size of the image produced by it, of an object of known size at a known distance, depending on its radius of curvature. The size of the image is determined by doubling it with a double refracting prism, and then altering the strength of the prism until the images come into contact. When this has taken place, a displacement equal to the size of the image has been produced. Two objects known as mires are situated upon an arc, one a white rectilinear figure, which is stationary, and the other, made up of white enameled blocks, capable of being moved along the arc. These objects are so placed that their images are reflected by the cornea and are viewed through a telescope by the observer. The telescope contains a prism (to double the images) placed between two bi-convex lenses, with a third bi-convex lens to shorten the posterior foci of the two images. It stands upon a tripod, which can be moved in order to obtain the proper focus. The patient places his chin on the rest and looks in the tube, the eye which is not under observation being covered by a disc.
When everything is properly adjusted, the central images are obtained on a spider web, also provided in the tube, and the movable object is moved along the arc until its image comes in contact with the image of the rectilinear object. Its position on the arc is noted upon the index.
The arc itself is now turned on its own axis to a position at right angles to its first position, and the relation of the two images noted, but if absent, they will occupy the same relative position. The degree of astigmatism is measured by the overlapping, each step of the white enameled block representing one dioptre. If the reflections are separated in the second position they are brought in contact once more and the arc turned back to its primary position, where they will overlap. The overlapping in this case will give the measure of an astigmatism against the rule; that of the previous instance with the rule. The ophthalmometer merely measures the corneal astigmatism and not the refraction of the eye. Corneal astigmatism may be modified by lental astigmatism, consequently the ordinary test with the trial lenses should always be made in addition.
Ophthalmometry is serviceable in revealing the corneal astigmatism and the principal meridians.
General Considerations.-After the test is completed a proper record of it should be made in the surgeon's case-book. As this is the first step in testing any case, the result forms an integral part of the complete record. To it should be added the record of the correcting lens, and finally the acuteness of vision produced by this lens. The following example will illustrate this:-
O. D. V = 5/8 plus 2.50 D V = 5/5
O. S. V = 5/5 plus 0. 75 D V = 5/5
When the correcting lens is a compound glass, the component lenses are united by the sign (), which signifies "combined with." Thus a combination composed of a plus 3. D spherical with a plus 1.50 D cylinder axis vertical is recorded plus 3. DS () plus 1.50 DC axis 90 degree. Notice that the spherical lens is always recorded in advance of the cylinder, and that the sign plus or - is expressed to show that the lenses are either convex or concave.
Simple cylinders are ground on one side only, the other side being plane. Compound glasses contain the spherical on one side and the cylinder upon the other. The correcting lenses for mixed astigmatism may either be crossed cylinders, a convex cylinder on one side and a concave cylinder at right angles on the other, or a combination of a spherical with a cylinder. Thus in a case of mixed astigmatism corrected by a convex cylinder of 2. D axis vertical and a concave cylinder of 1. D axis horizontal the prescription may either be plus 2. Dc axis 90 D - 1. Dc axis 180, or 2. Ds () - 3. Dc axis 180. The two are optical equivalents. Unless specially ordered to do otherwise, the optician will grind the lens according to the latter. The sign is usually employed when two cylinders are combined, the convex cylinder being expressed first.
Patients should be instructed to return with their glasses after they have obtained them from the optician, in order that the surgeon may ascertain if the lenses have been correctly ground and that the frames are properly fitted. The importance of this cannot be overestimated. The correctness of the lens is verified by neutralizing it with the opposite form of lens, that is,convex spherical and cylinders either alone or in combination are neutralized by concave sphericals or cylinders on the same number. If one holds a convex glass near one eye, and fixed an object like the edge of a door, the edge will appear to move in an opposite direction to the lens when the latter is moved from side to side. With a concave lens it moves in the same direction. If a convex and concave lens of equal strength, held together, are moved in a similar manner, the object will remain stationary, the effect of the convex being neutralized by the concave. Thus the number of any lens can be ascertained by neutralizing it with the lenses from a trial case, which are always numbered.
The optical centre of the lens should coincide with its geometric centre, unless it has been ordered decentered. To find the optical centre of a lens we can utilize the reflection of a piece of paper pasted on the window pane with our back turned to the window. A reflection of it appears on the anterior and posterior surfaces of the lens and when the images overlie each other the optic axis of the lens is determined and therefore the optical centre which is situated upon this. The point can be marked upon the lens with ink and its situation as regards the geometric centre noted.
It can also be determined by refraction by finding two meridians of the lens at right angles to each other which do not displace a vertical line horizontally when it is viewed through the lens. It is customary to take the edge of a door or a card. When the lens occupies such a position that the vertical line appears as a continuous line, above and below, with that seen through the lens, this meridian is marked by a line with a pen. The glass is rotated at right angles and the same process gone through with again. The point at which these two lines intersect each other is the optical centre. The frames should be so fitted that the pupil is opposite the geometric centre, and the optical centre should coincide with the latter unless it has been purposely decentered. This is the rule when glasses are to be worn for distance of constant use. Near glasses are usually decentered in about 4 mm. on account of the convergence of the visual lines. Reading glasses are also tilted forward and placed lower down than distance glasses, to conform with the depression of the visual line. A decentered glass when not required is sufficient to render void the beneficial effects of the most perfect prescription. Lenses are often decentered in a given direction when a prismatic effect is desired. Convex and concave lenses may be considered as being made up of a series of prisms. The effect of decentering a convex glass in, is equivalent to obtaining a prism with its base in ,while to decenter a concave glass inwards produces a prism with its base out. The degree of prismatic effect obtainable depends on the distance the lens is decentered and the strength of the lens. The greater the decentration and the greater the strength of the lens the greater the prismatic effect.
Spectacle frames are always preferable to eye-glasses, and in fact their use is imperative in high grades of astigmatism. Still the prejudices of many patients, particularly women, against them must be regarded if we wish them to wear their glasses, so under these circumstances eye-glasses must frequently be prescribed. Nowadays, with the many improved guards in use, eye-glasses can be fitted nearly as perfectly as spectacles, and if the patient is taught to take proper care of them and to have them readjusted frequently they answer the purpose quite as well.
When separate glasses for reading and distance are required a "bifocal lens" for constant use may be prescribed to avoid the inconvenience of changing from one to the other. Many people never become accustomed to them, however, and often meet with accidents caused by looking through the lower part of the glass when going down stairs or stepping out of a conveyance.
By dioptometry is understood the methods for determining the refraction and accommodation of the eye. These methods are of two kinds-subjective and objective.
SUBJECTIVE DIOPTOMETRY embraces the methods which depend largely upon the statements of the patients themselves.
The method which is almost universally used, and which it is wise always to employ even though other methods are also followed, is that based upon the acuteness of vision.
It has been determined by experiment that the smallest distance separating two objects which permits of their being seen discrete is one that subtends a visual angle of one minute. Nearer than that they appear as one. The visual angle may be conceived to be formed by lines extending from the extremities of an object which meet at the nodal point of the eye, as in Fig. 20.
These lines represent secondary axes, which cross each other at the nodal point without undergoing refraction, and upon reaching the retina determine the size of the retinal image.
Snellen's test-types which are in general use are based upon this principle. Each letter as a whole, held at the distance marked above it, subtends an angle of 5', while the component strokes and the spaces between contiguous strokes subtend angles of I.
It is evident by the figure that the distance of the object is an important matter. The size of the object remaining the same, the angle becomes larger the nearer the object is brought to the eye; while conversely, the greater the distance the larger the object must be to preserve the same angle. Snellen's test- types are so designed that they are seen under a visual angle of five minutes when held at the distance at which they should be seen. The largest type should be seen at 60 metres by the normal eye, and from this they range down to a size visible at five metres. Fig. 22 shows them reduced in size. In testing the acuteness of vision, which is the first step to be taken, the patient should be seated with his back to the light and the test- type for distance placed opposite him at a distance of five metres or more, as space will permit. Such a distance is practically, infinity, and has the advantage that such rays which come from the card and enter the eye are parallel. Testing each eye separately, the patient is asked to read the smallest line of letters he can. His acuteness of vision (V) is expressed by a fraction, the numerator of which represents the distance of the test-card and the denominator, the distance at which the line of type he read should be distinguished. Thus, if he simply read the largest type at a distance of five metres his acuteness of vision would be expressed as follows.
V=5/60 It is important not to reduce the fraction, as it represents both the distance and the line read.
The abbreviations O.D. and O.S. respectively stand for the right and left eye, and are utilized for designating the eye examined. The abbreviation O.U. stands for both eyes used simultaneously. Should the patient's sight be so bad that he is unable to read the largest type, the greatest distance at which he can count the examiner's fingers should be ascertained. If even this is impossible, he should be placed in a dark room, and by alternately shading and uncovering a lighted candles his power to distinguish light should be noted.
After the acuteness of vision has been ascertained and recorded, the next step is the determination of the static refraction. In order to do this, it is necessary to possess a case of trial lenses and appurtenances, such as can be found at a first-class optician's, and several trial frames. The numbering of lenses is now almost universally after the metric system which takes as the unit a lens having a refractive power of 1 dioptre, and which has a focal length of 1 metre, or about 40 inches.
A lens of 2. D. is twice as strong and, therefore, has a focal distance of half a metre. Between the whole numbers are lenses of .25 D .50 D. and .75 D. The advantage of this system over the old or English system, where a strong lens was taken as the unit and where the number expressed the focal distance and not the refractive power,is that we are dealing with whole numbers in our calculations and not with vulgar fractions. It is a very simple matter both to find the focal distance of a given lens of the dioptric system and its equivalent in the English system.
If it be required to find the focal distance of a given lens of the dioptric system, divide 100 centimeters (1 metre) by the number of the lens and the answer will be the focal length in centimetres. For example, the focal length of 5 D. 100/5 = 20 cm. If the focal length is known and we desire to ascertain its dioptric number, we divide 100 cm. by the focal length, as for example with a focal length of 20 cm., thus 100/20 = 5. D.
In translating from the old inch system to the metric, we can consider to inches equal to one metre, and to obtain its dioptric equivalent, we divide 40 by the number of the lens in inches. For instance, No. 20 of the old system is equal to 2. D., for 40/20 = 2.
To convex lenses is given the plus sign (Plus), and to concave lenses the minus (-) sign.
In ascertaining the static refraction, each eye must be tested separately, as in the case of the acuteness of vision. Considerable advantage is obtained by commencing the test with convex spherical lenses, as these cannot be overcome by an effort of the accommodation. If these lenses increase the acuteness of vision, or do not make it worse, the refraction is hyperopic. Should the weakest convex lenses make the vision worse, concave spherical lenses should be employed. In the event of their failure to improve, convex cylindrical lenses are next utilized, and lastly concave cylinders. Even though the acuteness of vision is normal in the first place, it is still necessary to place convex lenses in front of the eye in order to determine if there is any manifest hyperopia present. Under such circumstances the strongest convex lens through which the said line of type can be read is the measure of the manifest hyperopia. In some instances the acuteness of vision may not be up to the normal, and no lens or combination of lenses makes it so, though the same line can be read equally well with convex lenses up to a certain strength. In this case the strongest lens also represents the manifest hyperopia. If convex lenses improve the vision to a certain degree, but short of the normal, recourse should next be had to convex cylindrical lenses in addition to the strongest sphericals found, which may bring it up to normal, the case being one of compound hyperopic astigmatism. The cylinder must be rotated in front of the spherical until the axis of the astigmatism is found. The strongest convex cylinder should be ascertained as in the case of convex sphericals.
In the event of failure with convex glasses, concave ones should be employed. Unless they actually improve the vision they are not to be considered, because all eyes, no matter what their refraction is, can overcome the weaker concave lenses by an effort of the accommodation. Presuming, however, that they do improve the vision, the weakest concave glass which produces the maximum acuteness obtainable is the measure of the myopia.
If the vision is improved somewhat by concave glasses, though not up to normal, concave cylinders should be tried in addition to the weakest spherical obtained in the first place, and the combination may bring the vision up to normal. This would indicate compound myopic astigmatism.
Failing with both convex and concave sphericals, convex cylinders should be employed to find if simple hyperopic astigmatism exists. The cylinder should be slowly rotated in the frames in order to find at what axis it seems best. This being found, stronger lenses are placed in the frames at this axis until the maximum improvement is obtained.
In the case of simple hyperopic astigmatism the strongest convex cylinder represent the measure of it.
Simple myopic astigmatism is tested in a similar manner, but here the weakest concave cylinder is the measure.
In testing as if for simple hyperopic astigmatism, a certain improvement may be obtained, but less than normal. Leaving the strongest convex cylinder so ascertained in position, concave cylinders are added in a position at right angles to the former until the maximum improvement is obtained.
Such a combination composed of the strongest convex cylinder and the weakest concave cylinder is the measure of the mixed astigmatism.
This, in brief, is the plan to be followed in the examination of any given case, and if closely adhered to will prevent much confusion and loss of time. There are other methods of determining the astigmatism which must also be employed either as soon as its presence is determined or as a check upon results obtained after the former methods. Thus the presence of astigmatism is frequently discovered by asking the patient to look at the clock-face test- type, made up of lines, in series of three, radiating from a center in various directions. Wallace's chart is one of the most convenient. If astigmatism is present, one set of lines will stand out clear and distinct, while the others, but especially those at right angles to the first, will be indistinct. These designate the principal meridians of the astigmatism. Or the stenopaic slit may be rotated in front of the eye until a point is found where the vision is most distinct, which will designate one of the principal meridians, and as the emeridians are always at right angles to one another the other meridian is determined at the same time. Convex and concave glasses are now placed in front of the slit and the degree of hyperopia or myopia, if either exists, ascertained. Next, rotating the slit to a position at right angles to the first the same procedure is again followed out. If convex lenses improve or do not make the vision worse in one meridian, and concave lenses fail to improve it in the other, the case is one of simple hyperopic astigmatism. If concave glasses improve the vision in one position, and convex glasses make it worse in the other, simple myopic astigmatism is present.
If convex glasses improve or do not make vision worse in both positions, it is a case of compound hyperopic astigmatism. The difference between the strongest convex glass in each position represents the astigmatism, and the weaker of the two, thus found, the hyperopia. Compound myopic astigmatism is determined in the same manner by the difference between the two weakest concave glasses.
If the case is mixed astigmatism, convex glasses will improve or will not make the vision worse in one position and concave glasses will improve the vision in the other.
Numerous combinations and variations of these methods are made by different surgeons, but the same principles hold good throughout.
After the astigmatism is determined by any of these methods, it is usual to place the correcting lenses in the frames and have the patient look at the clock face, when, if the astigmatism is properly corrected, the lines will all appear similar.
In all cases of astigmatism, or in any case where spasm of the accommodation is found or suspected, the test should also be made under the influence of a cycloplegic.
Cycloplegics.-By Cycloplegics are meant those drugs which produce temporary paralysis of the ciliary muscles, and therefore suspension of the accommodation. The importance of this in determining ametropia has been stated in the preceding chapter. In addition, however, complete physiological rest of the eyes is obtained which often removes congestive conditions of the retina and choroid, and later when glasses are prescribed they give more comfort than they would have done without the use of a cycloplegic. The drugs most commonly employed are the sulphates of atropine, hyoscyamine and duboisine and the hydrobromates of homatropine and scopolamine.
(1) Atropine is usually employed in a strength of four grains to the ounce. Ordinarily it paralyzes the accommodation in about two hours and the effects remain for a week. A drop of the solution should be dropped into the outer canthus three times during one day. In cases of marked spasm of the accommodation in young hyperopic subjects it can be continued for several days.
(2) Hyoscyamine and Duboisine are employed in the form of solutions made up of two grains to the ounce. Their action is much more rapid than atropine and their cycloplegic effect more transitory.
(3) Scopolamine may either be employed in a one per cent. solution, a single drop being instilled, or in a one-fifth per cent solution, one drop every fifteen minutes for an hour and a half. Cycloplegia occurs in about forty-five minutes and lasts from three to five days. Toxic symptoms sometimes develop, so considerable care should be exercised.
(4) Homatropine used in a three per cent. solution, one drop being instilled every fifteen minutes for an hour and a-half preceding the examination, can be employed when a very transitory effect on the ciliary muscle is desired. Its effect is increased by dropping a drop of a four per cent. solution of cocaine in the eye each time before instilling the homatropine. The cycloplegic effects of homatropine pass off in about fifty hours, and are in a degree neutralized by eserine. It is not safe to use strong cycloplegics in elderly people on account of the danger of precipitating an attack of glaucoma. Of course, they must never be employed if glaucoma is suspected or present. It is unnecessary to use them in people whose advanced age denotes that the accommodative power is very weak.
Patients soon become familiar with the letters on a test card, and children are apt to memorize them before being tested, so it is advisable to have several cards with different letters. Thus a new card should always be displayed for each eye, and if at any time there is any suspicion that the patient is drawing upon his memory a different one must be substituted. In order to avoid the necessity of walking across the room each time to make such a change, and especially in order to be able to make it without the patient's knowledge, I devised a changeable test- type, the plans of which were presented to Messrs. Clairmont & Co., of New York, who made the apparatus and perfected the motor for working it. This instrument was described in a paper read before the Homoeopathic Medical Society of the County of New York in 1893.
It is illustrated in Fig. 22.
The apparatus consists of an ornamented wooden case, upon which are mounted the ordinary Snellen test-types. The five lower lines only are capable of being changed, the changes being produced by the revolution of five quadrangular rollers permitting the exhibition of four series of letters. Motor power is furnished by an accordion-pleated rubber tuber, which, when expanded by the column of air communicated to it by the pressure of a bulb, elevates a weight to which is attached an arm. As the arm moves upward it carries a cog which locks with the wheel that revolves the five rollers. This wheel contains four slots, placed at intervals of ninety degrees, for the reception of two catches, an upper and a lower one, which limit the revolution of the wheel to a quarter of a circle. After each quarter revolution the weight carries the arm back to its former position, setting the apparatus for the next change. It is operated by a bulb at the side of the patient, which is connected with the motor by tubing.
An instrument called the Refractometer has been invented by H.L. De Zeng, whose purpose is the estimation of the total refractive error and particularly the whole amount of astigmatism present in all the dioptric media without the use of a cycloplegic.
This instrument shown in Fig. 23 is manufactured by the Cataract Optical Co., of Buffalo. While disclaiming that any mechanical device can wholly replace the ordinary test under a cycloplegic this instrument is certainly of great value where a cycloplegic is contra-indicated or refused, as well as in general routine work.
In brief this instrument consists of a nickel tube, in the head of which is placed a stationery concave lens of 20. D. It also contains an inner tube which is movable along its cylindrical axis by means of a rack and pinion adjustment, and which carries at its front end a convex achromatic objective. These lenses in combination at different distances give all the spherical foci from plus .12 D to plus 18. D, and from - .1 D to - 0. D inclusive. The convex spherical effects are recorded upon a revolving dial at the side and the concave effects upon the top of the inner tube, visible to the observer through an oval opening in the top of the outer tube.
Owing to the range of the negative scale being limited to-9. D, two auxillary caps accompany the instrument, one containing 10. D and the other-20. D, which, when placed over the eye piece raises the negative scale to either-19. D or-29. D respectively. The outer tube is further armed at its front end with a revolving head, composed of two revolving discs, containing blanks, a stenopaic slit, and eleven minus concave cylinders set at right angles with their radii. The resulting combinations possible give a range of cylindrical lenses from-.12 D to-8.75 D, which can be rotated to any given axis, the latter being indicated.
By reason of the instrument's optical construction, it has an amplification of two and one-third diameters, and in consequence of this the test-types furnished with it are reduced to three-sevenths of the size of Snellen's letters, so that the visual acuity may be reliably estimated with the instrument. The instrument must be properly adjusted for whatever range is desired, either 3,4,5 or 6 metres.
The best method employed in testing is what is known as the "fogging system," which consists in over-correcting a hyperopic eye with convex lenses or under-correcting a myopic one with concave lenses which are too weak. The effects of this is to render the lines and letters deeply blurred, which causes relaxation of the ciliary muscle and in consequence latent errors to become manifest. With the instrument properly adjusted, and the patient properly placed, the thumb-screw is turned until the test-letters are distinctly seen and the reading noted.
Fogging is next resorted to by producing artificial myopia, which encourages relaxation of the accommodation. The thumbscrew is now tuned slowly back and the patient requested to watch the astigmatic fan. If he states that one or more lines appear distinct before the others he is astigmatic, whereas if they appear equally distinct simultaneously the error is simple hyperopia or myopia. If astigmatism is present it is necessary to utilize the concave cylinders contained in the revolving discs to render the lines equally clear. This procedure should be repeated to verify the result or to make any necessary corrections.
The power of convergence is tested with an ophthalmo- dynamometer, that of Landolt's being the simplest and best. It consists of a metallic cylinder blackened on the outside, containing a vertical slit 0.3 mm. wide covered with ground glass. Beneath the cylinder is attached a tape measure graduated on one side in centimetres, and on the other in metre-angles. The vertical line of light, produced when the cylinder is placed over a lighted candle, is the object of fixation. Approaching the patient in the median line until the patient sees the line double the near point of convergence is found and the distance in centimetres with its equivalent in metre-angles recorded. The minimum of convergence is found by withdrawing the instrument from the patient; but as it is usually negative, it is determined by the strength of the strongest abducting prism which will not cause diplopia with the patient fixing the object at six metres. If the number of this prism is divided by seven, the quotient will approximately give in metre angles the deviation of each eye when the prism is placed before one. The amplitude of convergence is equivalent to the difference between the maximum and minimum of convergence.
Accommodation is tested by means of the reading types of Snellen or Jaeger. A sample is shown in Fig. 24.
It is necessary to test each eye separately, and finally both together. The nearest point the type can be read represents the punctum proximum. This subject has already been discussed in the preceding chapter.
Presbyopia is to be determined after the static refraction has been tested. With the distance glasses in the frame, the patient is asked to hold the reading type or a newspaper at the distance at which he desires to work or read. Convex glasses are now added to the distance glasses, until the best vision at this distance is obtained. The optical equivalent of the glasses before each eye is now computed, and the resultant glass is presumably his prescription for near. The approximate amount of presbyopia at different ages can be computed from the chart (Fig.10) given in the previous chapter. This amount must be added to the amount of hyperopia and deduced from myopia. This should not be considered as final, but used simply as a check on the result obtained by testing and to save unnecessary loss of time.
OBJECTIVE DIOPTOMETRY.- Objective dioptometry embraces the methods of determining the refraction independent of any statements by the patient. These methods are valuable in conjunction with the test made by the trial lenses and test letters, and especially so when the patient is illiterate or too young to read letters.
Frequently they are utilized to save time in arriving at an approximate estimate of the error, the accurate degree of which is fully determined in the ordinary manner. Not that they are inaccurate in themselves when applied by an expert, but because it is a safer plan to check results.
Estimation of Refraction by the Direct Method- A qualitative estimation can be made with the ophthalmoscope held at a little distance from the observed eye. When it is remembered that the rays issuing from a hyperopic eye are divergent and those from a myopic eye convergent, it is easy to understand how an observer can see an upright image of the retinal vessels in the former and an inverted aerial image in front of the latter. If the observer moves his head from side to side, the vessels seen in a hyperopic eye will move with the mirror, the image being upright, while they will move in the opposite direction if the eye be myopic. At this distance no vessels can be seen in an emmetropic eye, because the pencils of rays emanating from any two points upon the retina (each is made up of parallel rays) will diverge from each other so that no rays will enter the observer's eye. Close to the eye, however, an upright image of the fundus can be seen. The quantitative examination is conducted with the mirror held close to the observed eye, if possible as near 13 mm., the anterior focus of the eye and the proper situation for the glasses to be worn. If the examination is conducted at a greater distance proper allowance must be made. In following out this method, it is imperative that both the surgeon and the patient thoroughly relax their accommodation. This is easily accomplished for the patient, if no spasm exists, when the examination is made in a dark room, but the examiner can only attain it after much practice. It is rather doubtful if any expert can relax his accommodation absolutely unless he be so old that he practically has none. Still, many can do so thoroughly enough to obtain approximately correct tests.
The most desirable point in the eye-ground upon which to focus is either the edge of the disc or the medium-sized vessels between the disc and the macula, especially two vessels running at right angles to each other. The macula is unsuitable, because of the contraction of the pupil caused by throwing the light upon it and the annoying corneal reflections obtained.
If the observer has any error of refraction, it must either be corrected with glasses or allowance made for it in computing the final result. all these conditions being fulfilled the examination is commenced. If the patient's eye is emmetropic, the vessels will be seen distinctly, and convex lenses rotated back of the opening in the ophthalmoscope will blur them. If hyperopia is present the divergent rays issuing from the eye are rendered parallel by the rotation of convex lenses, and the strongest convex lens through which the vessels are seen distinctly is the measure of the error. The convergent rays from a myopic eye are rendered parallel by concave glasses, and the weakest concave lens through which the vessels are seen most distinctly is a measure of the myopia. The direct method is used to determine the height of retinal tumors by estimating the refraction at their summit, and the depth of a cupping of the papilla of estimating the refraction at its bottom.
If the examination is conducted at a distance not more than 2.5 cm. from an eye, a hyperopia of 1. D will represent a shortening of the axis equal to 0.3 mm. and a myopia of 1.D a lengthening of the axis to that same amount.
Astigmatism can also be estimated by this method. It is known to be present when the upper and lower margins of the disc and the horizontal vessels are well defined while the lateral margins and vertical vessels are blurred, or vice versa.
Its presence may also be suspected if the disc is elongated either horizontally, or vertically, the long axis corresponding to the meridian of greatest refraction.
In estimating the degree it is the best to fix tow vessels running at right angles to each other and whose direction conform most nearly to the principal meridians of the astigmatism. If the vessels in one meridian are seen distinctly, while in order to see the vessels in the opposite meridian a convex or concave lens is necessary, the case is respectively one of simple hyperopic or myopic astigmatism. If, in order to see distinctly the vessels in the two principal meridians, two convex or two concave lenses of different strength is required, the case is one of compound astigmatism, either hyperopic or myopic. The difference in strength between the two convex or two concave lenses represents the astigmatism, while the weaker lens represents the simple hyperopia or myopia. The axis of the cylinder to correct the astigmatism is placed in the direction of the vessel which was seen through the strongest of the two lenses.
It seems unnecessary to again state that in hyperopic astigmatism the strongest convex lenses represent the measure and in myopic astigmatism the weakest concave.
In mixed astigmatism the vessels in one meridian are rendered distinct by convex glasses and the vessels in the opposite meridian by concave glasses. Hyperopia exists in the meridian at right angles to that in which the vessels were made distinct by the convex glasses and myopia exists in the other. The axes of the cylinders to correct this would be the reverse of this, because they refract only those rays which enter at right angles to their axes.
Estimation of the Refraction by the Indirect Method.- This method is not generally resorted to, because of its difficulties. It is sometimes used in estimating very high degrees of myopia, but rarely in hyperopia. The rays coming from an emmetropic eye being parallel they are brought to a focus by the object glass at its principal focus, whereas the divergent rays from a hyperopic eye are brought to a focus farther from the object glass than its principal focus and the convergent rays from a myopic eye nearer than its principal focus. The degree of the ametropia is determined by measuring this distance. Schmidt-Rimpler's method is usually employed.
Schweigger uses the indirect method in comparison with the direct method to determine the presence of astigmatism. It has already been explained that the disc appears elongated in the direction of the meridian of greatest refraction when seen in the upright image, so it is only necessary to state that the reverse obtains in the inverted image.
Skiascopy (Retinoscopy, or the Shadow Test).- This is a method of determining the refraction by observing the direction in which the light appears to move across the pupil when the mirror is rotated in various directions. The test should be made in a dark room at a distance of one metre from the patient. It is convenient to place a tape on the wall extending from the position of the examined eye to a distance beyond that of the observer. This is graduated in centimetres, and at appropriate intervals the corresponding number of dioptres marked. Either a plane or concave mirror can be employed, but the preference is with the former. The light is covered with an opaque asbestos shade having an aperture 1 cm. in diameter. If the plane mirror is used the light should be near the surgeon, but if the concave mirror is used, behind the patient. The arrangement is shown in Fig. 25.
Skiascopy has been elaborated especially by Jackson, whose description is here followed and whose work is the ablest published.
By rotating the mirror the area of light it throws on the face is made to move in the direction the mirror is rotated. Those rays which enter the pupil from a small light area on the retina, which also moves when the mirror is rotated. This area moves with the light on the face when the plane mirror is used and in the opposite direction if the concave mirror is employed.
For the plane mirror this movement is shown in Fig. 26.
When the mirror occupies the position A A the rays from L which enter the eye are reflected as if they came from L, and after passing through the eye are condensed at a, on the lower part of the retina. At the same time the light falls on the lower part of
Skiascopy by Dr. Edward Jackson. the face. If now the mirror is rotated to B B the light which enters the eye is reflected as if it came from l, and is condensed toward b, on the upper part of the retina. Simultaneously the light which falls on the face moves upward also. The same movement of the light with the light on the face occurs in hyperopia and myopia as well as in emmetropia.
The movement of the light area on the retina caused by a concave mirror is shown in Fig. 27.
When the mirror occupies the position A A the rays which enter the eye come from the focus of the mirror at l and are condensed towards a, on the upper part of the retina, while the light falls on the lower part of the face. When the mirror is rotated to B B the rays which enter the eye come from l and are condensed towards b, on the lower part of the retina, while the light on the face moves upward. The same is true in hyperopia and myopia. The real movement of the light on the retina, as it would appear from behind, is always with the light on the face with the plane mirror and in the opposite direction with the concave mirror.
In our examination, however, we do not see it in that way, but we watch the apparent movement as seen through the pupil. When the plane mirror is used the apparent movement in the pupil and the real movement on the retina are the same when the observer sees an erect image, and in the opposite direction when he sees an inverted image.
The reverse of this obtains with the concave mirror.
The rays of light coming from a myopic eye are convergent and cross each other at its far point, and proceed divergently. The point B (Fig. 28), at which they cross and which corresponds to the far point, is known as the point of reversal. As has been explained under myopia, the distance of the far point from the eye represents the focal length of the glass required to correct it, and, therefore, if the point of reversal is known the amount of myopia is also known.
Retinoscopy is an accurate method of determining the point of reversal.
In the following description it is assumed that the plane mirror is used, though it will apply equally to the concave mirror if we reverse the movement in the pupil and change the lenses. If the mirror is held closer to the eye than its point of reversal, as at A (Fig.28 ), an erect image is seen, and the light in the pupil will seem to move with the light on the face. Beyond the point of reversal, as at C, an inverted image is seen, and the light will appear to move in an opposite direction to the light on the face.
At the point of reversal it is impossible to see which way the light moves, and the illumination is much more feeble. At a distance of one or two metres from the point of reversal the illumination is very bright, but as the distance increases it becomes more and more feeble. Without altering the rapidity of the movement of the mirror, the apparent movement of the light is more rapid as we approach the point of reversal.
While the test depends mainly on the direction of the movement of the light in the pupil, the degree of illumination and the rapidity of movement aid in arriving at a diagnosis.
Myopia is estimated by finding the nearest point that an inverted image is seen (C, Fig. 28 ) and the most distant point at which an upright image is seen, as at A. Midway between the two is the point of reversal, B, whose distance from the eye should be noted on the tape for that purpose, and the number of dioptres corresponding is the measure of the myopia. Thus if C is at 55 cm. and A at 45 cm., which corresponds to 2. D. giving a myopia of that amount. When the myopia is of a high degree, the point of reversal lies very close to the eye, and in this situation a slight error in marking the distance may mean an error of a dioptre or more in estimating the myopia; whereas if a similar error is made when the point of reversal is situated at a metre or more from the eye it is unimportant. Therefore, in high degrees of myopia, in order to check results, it is well to correct all but about one dioptre by placing a suitable concave lens in a frame before the eye and measure the remainder, which is to be added to the lens in the frame.
If the myopia is less than one dioptre, the point of reversal lies so far away from the eye that when near it one cannot see which way the light moves. In this case put a weak convex glass in the frame to increase the myopia, then determine the point of reversal, and deduct the convex glass from the amount of myopia found.
Hyperopia gives an upright image no matter how far we recede from the eye, because the rays leave it divergently. In order to obtain a point of reversal, it is necessary to convert it into an artificial myopia by putting a convex glass in the frame and then finding the point of reversal as in myopia. The amount of myopia is to be deducted from the convex lens, the hyperopia being represented by the remainder.
Thus, if a convex 5. D lens is placed in the frame, and the point of reversal is found at 1 metre (1. D), the hyperopia will be 4. D. That is 4. D out of the five were necessary to render the divergent rays parallel; and the other dioptre to bring them to a focus at 1 metre.
Emmetropia acts the same as hyperopia, but when a convex lens is added the myopia produced equals the strength of the lens, proving that the rays were parallel in the first place. In astigmatism the refraction of the principal meridians is obtained in the same way as in myopia or hyperopia. In order to determine the refraction of a certain meridian, it is necessary to rotate the mirror about an axis at right angles to it, which causes the light to traverse the length of the meridian. The direction of the meridian is revealed by the areas of light assuming a band-like shape as its point of reversal is approached. This is most marked in the higher degrees of astigmatism. Near the point of reversal, where the band-like appearance is most distinct, it is easy to cause the apparent movement from side to side, but more difficult in the direction of the length of the band. The latter, however, is to be watched, as it determines the point of reversal.
If the astigmatism is of low degree, this band-like appearance may not be perceptible; but when we have determined the point of reversal of one meridian it will be found that there is still distinct movement, either upright or inverted, in the direction at right angles to this. Supposing the case to be one of myopic astigmatism, either natural or artificial, and the surgeon starts at a point nearer the eye than either point of reversal and gradually withdraws from it, the following phenomena occur: At the start the movement will be with the light on the face in all directions. Withdrawing to the nearest point of reversal there will be no movement in the meridian whose reversal-point it is, but direct movement at right angles to it. Between the two points of reversal the former meridian gives an inverted movement and the latter direct. At the farthest reversal-point the direct movement for the meridian ceases and the other remains indirect. Beyond both points of reversal the movement is against the mirror in all directions. The degree of the myopia of both principal meridians, either natural or artificial, having been determined,the astigmatism present is represented by the difference between them. As a final test the cylinder correcting the astigmatism should be put in the frame together with the concave or convex lens which will remove the point of reversal to about 1 metre, and the movements watched again.
When using the concave mirror the position of the observer does not admit of much change, the distance being generally one metre. The movements of the light are the reverse of those just described. Skiascopy is the most valuable of all objective methods, and the student should practice it industriously upon an artificial eye made for the purpose before he can depend upon his results in actual practice.
Ophthalmometry.-The term indicates mensuration of the eye, but it is usually employed to mean the measurement of the radii of curvature of the cornea and the corneal astigmatism present in an eye with the ophthalmometer. The ophthalmometer in general use in this country is that of Javal and Schiotz and is shown in Fig. 29.
The principles upon which it is based are as follows: The surface of the cornea acts as a convex reflector, the size of the image produced by it, of an object of known size at a known distance, depending on its radius of curvature. The size of the image is determined by doubling it with a double refracting prism, and then altering the strength of the prism until the images come into contact. When this has taken place, a displacement equal to the size of the image has been produced. Two objects known as mires are situated upon an arc, one a white rectilinear figure, which is stationary, and the other, made up of white enameled blocks, capable of being moved along the arc. These objects are so placed that their images are reflected by the cornea and are viewed through a telescope by the observer. The telescope contains a prism (to double the images) placed between two bi-convex lenses, with a third bi-convex lens to shorten the posterior foci of the two images. It stands upon a tripod, which can be moved in order to obtain the proper focus. The patient places his chin on the rest and looks in the tube, the eye which is not under observation being covered by a disc.
When everything is properly adjusted, the central images are obtained on a spider web, also provided in the tube, and the movable object is moved along the arc until its image comes in contact with the image of the rectilinear object. Its position on the arc is noted upon the index.
The arc itself is now turned on its own axis to a position at right angles to its first position, and the relation of the two images noted, but if absent, they will occupy the same relative position. The degree of astigmatism is measured by the overlapping, each step of the white enameled block representing one dioptre. If the reflections are separated in the second position they are brought in contact once more and the arc turned back to its primary position, where they will overlap. The overlapping in this case will give the measure of an astigmatism against the rule; that of the previous instance with the rule. The ophthalmometer merely measures the corneal astigmatism and not the refraction of the eye. Corneal astigmatism may be modified by lental astigmatism, consequently the ordinary test with the trial lenses should always be made in addition.
Ophthalmometry is serviceable in revealing the corneal astigmatism and the principal meridians.
General Considerations.-After the test is completed a proper record of it should be made in the surgeon's case-book. As this is the first step in testing any case, the result forms an integral part of the complete record. To it should be added the record of the correcting lens, and finally the acuteness of vision produced by this lens. The following example will illustrate this:-
O. D. V = 5/8 plus 2.50 D V = 5/5
O. S. V = 5/5 plus 0. 75 D V = 5/5
When the correcting lens is a compound glass, the component lenses are united by the sign (), which signifies "combined with." Thus a combination composed of a plus 3. D spherical with a plus 1.50 D cylinder axis vertical is recorded plus 3. DS () plus 1.50 DC axis 90 degree. Notice that the spherical lens is always recorded in advance of the cylinder, and that the sign plus or - is expressed to show that the lenses are either convex or concave.
Simple cylinders are ground on one side only, the other side being plane. Compound glasses contain the spherical on one side and the cylinder upon the other. The correcting lenses for mixed astigmatism may either be crossed cylinders, a convex cylinder on one side and a concave cylinder at right angles on the other, or a combination of a spherical with a cylinder. Thus in a case of mixed astigmatism corrected by a convex cylinder of 2. D axis vertical and a concave cylinder of 1. D axis horizontal the prescription may either be plus 2. Dc axis 90 D - 1. Dc axis 180, or 2. Ds () - 3. Dc axis 180. The two are optical equivalents. Unless specially ordered to do otherwise, the optician will grind the lens according to the latter. The sign is usually employed when two cylinders are combined, the convex cylinder being expressed first.
Patients should be instructed to return with their glasses after they have obtained them from the optician, in order that the surgeon may ascertain if the lenses have been correctly ground and that the frames are properly fitted. The importance of this cannot be overestimated. The correctness of the lens is verified by neutralizing it with the opposite form of lens, that is,convex spherical and cylinders either alone or in combination are neutralized by concave sphericals or cylinders on the same number. If one holds a convex glass near one eye, and fixed an object like the edge of a door, the edge will appear to move in an opposite direction to the lens when the latter is moved from side to side. With a concave lens it moves in the same direction. If a convex and concave lens of equal strength, held together, are moved in a similar manner, the object will remain stationary, the effect of the convex being neutralized by the concave. Thus the number of any lens can be ascertained by neutralizing it with the lenses from a trial case, which are always numbered.
The optical centre of the lens should coincide with its geometric centre, unless it has been ordered decentered. To find the optical centre of a lens we can utilize the reflection of a piece of paper pasted on the window pane with our back turned to the window. A reflection of it appears on the anterior and posterior surfaces of the lens and when the images overlie each other the optic axis of the lens is determined and therefore the optical centre which is situated upon this. The point can be marked upon the lens with ink and its situation as regards the geometric centre noted.
It can also be determined by refraction by finding two meridians of the lens at right angles to each other which do not displace a vertical line horizontally when it is viewed through the lens. It is customary to take the edge of a door or a card. When the lens occupies such a position that the vertical line appears as a continuous line, above and below, with that seen through the lens, this meridian is marked by a line with a pen. The glass is rotated at right angles and the same process gone through with again. The point at which these two lines intersect each other is the optical centre. The frames should be so fitted that the pupil is opposite the geometric centre, and the optical centre should coincide with the latter unless it has been purposely decentered. This is the rule when glasses are to be worn for distance of constant use. Near glasses are usually decentered in about 4 mm. on account of the convergence of the visual lines. Reading glasses are also tilted forward and placed lower down than distance glasses, to conform with the depression of the visual line. A decentered glass when not required is sufficient to render void the beneficial effects of the most perfect prescription. Lenses are often decentered in a given direction when a prismatic effect is desired. Convex and concave lenses may be considered as being made up of a series of prisms. The effect of decentering a convex glass in, is equivalent to obtaining a prism with its base in ,while to decenter a concave glass inwards produces a prism with its base out. The degree of prismatic effect obtainable depends on the distance the lens is decentered and the strength of the lens. The greater the decentration and the greater the strength of the lens the greater the prismatic effect.
Spectacle frames are always preferable to eye-glasses, and in fact their use is imperative in high grades of astigmatism. Still the prejudices of many patients, particularly women, against them must be regarded if we wish them to wear their glasses, so under these circumstances eye-glasses must frequently be prescribed. Nowadays, with the many improved guards in use, eye-glasses can be fitted nearly as perfectly as spectacles, and if the patient is taught to take proper care of them and to have them readjusted frequently they answer the purpose quite as well.
When separate glasses for reading and distance are required a "bifocal lens" for constant use may be prescribed to avoid the inconvenience of changing from one to the other. Many people never become accustomed to them, however, and often meet with accidents caused by looking through the lower part of the glass when going down stairs or stepping out of a conveyance.
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