1 82 OPHTHALMOLOGY FOR VETERINARIANS 



refracted ray — depends upon the strength or degree of 

 the prism. Prisms are numbered from J degree up. 



Spheric lenses — those cut from a sphere — are refrac- 

 ting lenses. The convex or plus spheric lenses — of 

 which the crystalline lens of the eye is a type — collect 

 rays of light at a point on the opposite side; while 

 concave or minus spheric lenses diverge rays of Hght 

 on the opposite side. Rays passing through the optical 

 center of a lens are not refracted. 



Fig. 37. — Principal focus of a convex lens. The parallel rays a, h, c, d 

 are refracted by the lens so as to unite at the point F on the axis P; the 

 ray P undergoes no refraction. F is the principal focus, (de Schweinitz, 

 "Diseases of the Eye.") 



The point at which parallel rays are collected is the 

 principal focus of the lens. The distance of this point 

 from the optical center of the lens depends upon the radii 

 of curvature and its index of refraction. Rays which 

 diverge, back again through the lens, become again 

 parallel. 



When rays come from an object nearer than ^'infinity" 

 — supposed to be about 20 feet— they diverge, and are 



