A STUDY OF THE SENSES 



303 



focus by moving the lens nearer to or farther from the 

 plate. 



In the eye, too, we have an arrangement similar to that 

 of a camera, since the convex surfaces of the cornea and 

 crystalline lens bring the rays of light to a focus on the 

 sensitive rods and cones of the retina. Since, however, it 

 is impossible for the lenses within the eye to be moved 

 backward arid forward, focusing or ac-com-mo-da'tion of the 

 eye must be accomplished in a different way j namely, by 

 altering the shape of the lens. 



A B 



cm. c.p 



FIG. 138. Changes in Lens in Accommodation. 



A = adjustment of lens for distant cm = ciliary muscle. 



objects. ch = choroid coat. 



B = adjustment of lens for near cp = ciliary process. 



objects. si = suspensory ligament, 



c = cornea. 



Accommodation of the Eye. If I look out through the lace 

 curtains of a window at a distant object, say a tree, I can 

 see the latter more or less plainly, but the meshes of the 

 lace appear blurred. When, on the other hand, I turn my 

 attention to the individual threads of the curtain, the tree 

 is no longer distinctly visible, and I become conscious, too, 

 of a feeling of muscular effort. What takes place within 

 the eye is probably this. The suspensory ligament is con- 

 stantly pressing on the surface of the crystalline lens, and 

 so tends to keep its outer surface more or less flattened. 

 This condition enables the lens to focus clearly on the rods 

 and cones of the retina images of distant objects; the 



