Chap. XXIX.] 



THE PHAROSCOPE. 



385 



convex lens in front of the crystalline. This is 

 practically accomplished by the lens itself being 

 capable of adjustment for varying distances, a 

 capacity termed the power of accommodation. It 

 consists of an ability to alter the convexity of the 

 lens. This is effected by the contraction of the 

 ciliary muscle, which relaxes the anterior ligament 



Fig. 173. Accommodation of the Eye. 



of the lens, permits the lens to bulge forwards 

 by its own elasticity, and thus increases its con- 

 vexitv. The figure shows on the side marked I 



/ o 



the position of the lens when the mechanism of 

 accommodation is in repose, and on the side marked 

 p the new position in accommodation. It is mainly 

 the anterior surface of the lens that takes part in 

 the process. Its curvature augments, and its radius 

 of curvature for the greatest amount of accommodation 

 is diminished from 10 to 6 mm. The posterior sur- 

 face of the lens practically does not alter. 



The phakoscope is an instrument devised by 

 Helmholtz for rendering visible the alteration in 

 curvature of the anterior surface of the lens. It is 

 shown in Fig. 174. It consists of a black box made 

 of pasteboard, of the triangular shape shown in the 

 figure, and mounted on a stand. In. the centre of 



