The Senses. 



301 



focal distance from the lens ; if too close long sight results, 

 if too far away short sight is produced. 



Accommodation. — We must now consider the question of 

 accommodation, and the manner by which it is brought 

 about. 



The near point of the eye lies at about 8 inches from it ; 

 the far point in the emmetropic eye is infinity. It is not 

 necessary, however, that the eye should be capable of 

 focussing, adjusting, or accommodating itself for any and 

 every range between 8 inches and infinity, for at a point a 

 little over 200 yards from the eye the rays of light for all 



Fig. 32. 



FAR 

 Diagram t< 



NEAR 



ILLUSTRATE ACCOMMODATION. 



C.P., ciliary process ; T, iris ; Sp.l., suspensory ligament ; l.c.m., longi- 

 tudinal ciliary muscle ; c.c.m., circular ciliary muscle ; c.S., canal 

 of Schlemra (Foster after Helmholtz). 



The left half represents the shape of the lens for viewing far objects, 

 and the right half that for viewing near objects. 



practical purposes are parallel, and no effort of accommoda- 

 tion is required. From S inches up to 200 yards the eye 

 has to be capable of focussing itself, and the mechanism by 

 which this is produced we will now study. 



The rays of light passing through a convex lens, will be 

 brought to a focus at a greater or less distance behind the 

 lens, depending upon its curvature ; thus, the thicker the 

 lens the shorter the focal length, and vice versa. It is 

 obvious, therefore, that any mechanism which can alter the 

 curvature of the crystalline lens of the eye, will adjust it for 

 distant or near vision ; such an arrangement exists. The 



