134 VETERINARY OPHTHALMOLOGY. 



his eye into their line. Tlien what happens? The 

 line occupied by these rays is the same that was taken 

 by the entering rays, and if no rays enter the eye, none 

 will emerge. And when you try to intercept the rays 

 coming /rom the eye so as to make use of them in view- 

 ing the fundus, you get in the path of the lines of light 

 which enter and of course your head intercepts them. 

 Consequently, having cut off the source of light, the 

 result is darkness. For example, a candle a couple of 

 feet from the eyes will give divergent rays, which will 

 enter the eye, be refracted and focus on the retina, 

 forming an image of the candle-flame. The rays will 

 undergo reflection, and being reflected back through 

 the lens, will be again refracted, and you will find at 

 the candle-flame an image of the fundus, and at the 

 candle-flame is one of the conjugate foci. Of course, if 

 you interpose your head between the eye and the candle, 

 the rays emanating from the candle will be cut ofl: and, 

 in place of the observed eye being illuminated it will 

 be in a shadow of your own head. If you try to look 

 from the other side of the flame, i. e., having the flame 

 between you and the patient's eye, you will be dazzled by 

 the flame, as it radiates light in all directions though 

 in straight lines. And there the matter stood until, 

 in 1851, Helmholtz, after careful study evolved the 

 Ophthalmoscope. "What was wanted was a some- 

 thing which would allow an observer to bring his 

 head into his own light. This the mirror, which is a 



