THE EYE 155 



Accommodation. A camera must be focused with 

 reference to the distance of the features to be brought 

 out in the photograph. If it has been used for a land- 

 scape and is next to be used for a portrait the plate must 

 be set farther back from the lens. The same problem 

 exists for the eye but it is not met by changes of depth. 

 Instead, the lens is made to assume a more pronounced 

 curvature, so far as its front surface is concerned, when 

 the attention is directed toward anything near at hand. 

 This is accomplished by the contraction of smooth 

 muscle distributed in the middle coat of the eyeball in 

 the region surrounding the iris. We cannot discuss 

 here the mechanics of the act. The normal eye is de- 

 fined as one which forms clear images of distant objects 

 without effort. The act of accommodation is for near 

 vision and has a limit which is easily discovered, the 

 shortest distance at which we can clearly see details. 

 When the attention is shifted to something far away the 

 adjustment is a passive one; we do not speak of ac- 

 commodation in this case but of the relaxation of 

 accommodation. 



It can be observed that when accommodation for near 

 vision is employed there is a contraction of the pupil. 

 This is explained as follows. In any lens that can be 

 made the central part is more satisfactory for the forma- 

 tion of an image than the marginal part. In micro- 

 scopes, telescopes, and cameras diaphragms are used to 

 limit the passage of rays to the central portion of the 

 lenses. The more convex a lens is, the more essential 

 this restriction becomes. Therefore, when the crys- 

 talline lens has been rounded for the purpose of ac- 

 commodation, it is desirable to make the pupil narrower 

 than it may be when the lens is less convex. In tech- 

 nical language, the small opening is said to " diminish 

 spherical aberration." 



The extent to which the lens can be made to change its 

 shape is greatest in childhood and is progressively 

 lessened with the passing of the years. A man with 



