VISION. 799 



surface of the cornea; another, which is in the middle, is direct, 

 larger, but not so bright, and is due to the anterior surface of the 

 lens, which acts as a convex mirror; and, finally, a third image, 

 small, inverted, and brilliant, due to the posterior surface of the 



Fig. 346. Diagram Showing Refraction by a Double Convex Lens. 



( GANOT. ) 



The incident ray (L-B) is refracted at the points of incidence (B), and 

 emergence (D), toward the axis (M-N-A), which it cuts at F. 



lens, which also acts as a convex mirror. If the person experimented 

 on looks fixedly at objects placed at different distances, the only 

 change in the three reflections which we mentioned will be found to 

 take place in that caused by the anterior surface of the crystalline 

 lens. This fact leads to the conclusion that the phenomena of 

 accommodation are dependent upon a change in the anterior surface 

 of the crystalline lens. 



Fig. 347. Concave Lens Diverging Parallel Rays of Light. 

 ( LAHOUSSE. ) 



In the act of accommodation, when the candle is brought nearer 

 the eye, the image due to the anterior surface of the crystalline lens 

 becomes smaller because the lens becomes more convex. 



The form and variations in form of the dioptric surfaces of the 

 eye can be measured by Helmholtz's ophthalmometer and the phako- 

 scope of Helmholtz. 



