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.
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.