To secure an image of the fundus of the eye, including the entrance of the optic nerve (optic papilla), the tapetum, the pigmentary surface and retina and vessels, accommodation must be made for the normal refraction of the eye of the patient, and even for that of the observer.
In the emmetropic (normal) eye, the rays leave the surface of the cornea parallel to each other and it may be possible for the observer to secure a good image on his retina, without the aid of lenses. In the myopic (short sighted) eye they assume a convergent course on leaving the cornea, and to secure a satisfactory image a biconcave or plano-concave lens must be interposed between the cornea of the patient and the eye of the observer.
In the hypermetropic (long sighted) eye, the rays diverge in leaving the cornea of the patient, and a convex lens must be interposed between this and the eye of the observer, in order that the rays may be focused on the eye of the observer.
To adapt the vision to the different eyes the modern ophthalmoscope is furnished with a series of lenses concave and convex, any one of which can be moved behind the hole in the mirror to suit the demands of the particular case.
To make a satisfactory examination the pupil should be dilated as for oblique focal illumination. A 1:200 solution of apomorphia may be instilled into the eye (a drop or two) and in 20 to 25 minutes a satisfactory dilatation will have been secured. The effect of the homatropin will usually have disappeared in twenty-four hours.
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.