Determination of Static Refraction.
This can be best done in the lower animals by determining the strength of the lens required to render clear the image of its fundus. By knowing the refracting power of the lens, we may ascertain what deviation from the normal refraction there is in the eye under observation.
In making this test the mirror of the ophthalmoscope must be brought closely to the eye of the patient—1 to 2 inches.
If in such a case and without the use of any lens a distinct image of the fundus is obtained, and if this is rendered less distinct by interposing the lowest convex lens in front of the eye of the observer, the eye is emmetropic.
If the ophthalmoscopic mirror without a lens gives an indistinct vision of the fundus, and if the image is rendered clear by interposing one of the convex lenses, the eye is hypermetropic. The strength of the convex lens, +1, +2 or +3, dioptrics will give the measure of the hypermetropia.
If, on the contrary, the ophthalmoscopic mirror gives an indistinct image of the fundus, which is rendered even more indistinct by the interposition of a convex lens, but is cleared up and rendered definite by a concave lens, the patient is myopic. The strength of the concave lens used will give the degree of myopia, –1 dioptric, –2 dioptrics, etc.
The tendency in the horse is constantly to slight long-sightedness, but the deviation is rarely found to be serious either in this direction or in that of astigmatism.