1 84 OPHTHALMOLOGY FOR VETERINARIANS 



The virtual focus of a convex lens is the point at which 

 rays meet in a backward direction on the same side of 



Fig. 40. — Principal focus of a concave lens. Parallel rays a, b, d, e, 

 after refraction by the concave lens L, are rendered divergent as if they 

 came from the point F on the axis c. The ray c is not refracted. F, the 

 principal focus of a concave lens, is virtual, (de Schweinitz, "Diseases 

 of the Eye.") 



Fig. 41.— Virtual image of a convex lens: C, D is the object; C, D' is 

 the virtual image, erect and magnified, (de Schweinitz, "Diseases of the 

 Eye.") 



the lens to which they diverge, when the point of these 

 divergent rays is nearer to the lens than its principal 



