YETERINArvY OPHTHALMOLOGY. 133 



of our 2i inch lens, we illuminate the interior, and can 

 study its every part. "Well, then, why not the same 

 with the eye ? The eyeball is not a box simply. It 

 contains a lens, and that is why. If you throw a 

 pencil of light into the eye it will be brought to a focus 

 by the lens. That is not the case in a simple box. 

 Tlie light has to come back again and emerge from 

 the eye. So if the lens (biconvex) brings the entering 

 rays to a focus, it does the same for those emerging. 

 (See figs. G and 7.) But the entering rays were parallel 

 and brought to a focus through the mediation of the 

 lens, whereas the emerging rays, coming from a 

 focus, were rendered parallel. Let us go a little 

 furtlier. Suppose divergent rays be the case, as 

 they will pass to the lens and on returning will be 

 converged and made to meet at a focus in front of the 

 lens. As the rays primarily were not parallel, but 

 divergent, the focus at which they meet after passing 

 through the lens will not be at the same distance, as 

 you see. They will be further than the focus for 

 parallel rays. If one of tlie foci be brought nearer the 

 lens the otlier will be further off and are called con- 

 jugate foci. Xow, please notice that although con- 

 jugate they maintain a certain distance between each 

 other, for as you approach one foci the other recedes. 

 So, all rays emanating from the eye take a direction 

 toward the conjugate focus, and if one attempts using 

 this ray to see the fundus he must necessarily bring 



