10 VETERINARY OPHTHALMOLOGY. 



will not be acted upon, but will pass through un- 

 changed, and this is the chief axis. 



The distance between the central point of the lens and 

 the focus is the focal distance. If the diiection of the 

 rays are reversed and tliey proceed from a luminous 

 point at the focus, the rays will emerge from the lens, 

 parallel. 



When the distance of the light from the lens is 

 equal to the focal distance, the focus will lie at the 

 same distance on the opposite side of the lens, or ticice 

 the focal distance. 



If the luminous point approach the lens, the focal 

 point is moved farther away. If the rays proceed 

 from a chief point on the chief axis between the lens 

 and principal focus, they will diverge on the opposite 

 side of the lens, and not come to a focus. (Fig. 7.) In 



I 



Fig. 7. 



ordinary lenses, the refraction is not equal in amount 

 at the center and periphery. 



Rays passing through the optical center are not re- 

 fracted, while those which pass near the center are less 



