BY 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.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.