THE PRINCIPLES OF VISION 183 



collected at a point on the opposite side of the lens, at a 

 greater distance from the optical center of the lens than 

 the principal focus. The nearer the object is to the lens, 



Fig. 38. — Conjugate focus of a convex lens. The two dots in the axis 

 represent the principal foci, one being marked F. Rays diverging from 

 O converge after refraction to the point F\ farther than the principal 

 focus. Rays from F' also converge after refraction to 0. and F are 

 conjugate foci, (de Schweinitz, "Diseases of the Eye.") 



the greater is the divergence, and the farther is the con- 

 vergence on the opposite side. These two points — the 

 point of divergence and the point of convergence — are 



Fig. 39. — Virtual focus of a convex lens. Rays from the point 0, 

 less than the principal focal distance, diverge after refraction as if they 

 came from the point V. V is the virtual focus of O. (de Schweinitz, 

 "Diseases of the Eye.") 



known as the conjugate foci. These points are at an 

 equal distance when the point of divergence is at a dis- 

 tance twice the focal distance of the lens. 



