330 Veterinary Medicine. 



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 ra3^s 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 con- 

 vergent course on leaving the cornea, and to secure a satisfactory 

 image a biconcave or plano-concave lense mu.st be interposed be- 

 tvv^een 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 ophthalmo- 

 .scope 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 i : 200 solution of apomor- 

 phia 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 ophtlialmo.scope must be 

 brought closely to' the eye of the patient — -i 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 dis- 

 tinct by interposing the lowest convex lens in front of the eye of 

 the observer, the eye is emmetropic. 



