Chap. 17 



RESPONSIVENESS THE SENSE ORGANS 



327 



Gland 



Aqueous 

 humor 



Retina 



Spot of clearest 

 vision 



Optic 



nerve 



Fig. 17.16. General structure of the human eyeball cut in a vertical section, from 

 the top of the eye downward. The blind spot is the place where the fibers from the 

 cells in the retina leave the eyeball and form the optic nerve. When the eye is 

 directed upon an object it is placed so that the image falls upon the fovea, the area 

 of clearest vision. 



that is, changes the focus of the rays from near or far objects by changes 

 in the curvature of the crystalline lens. This is made possible by stretching 

 or relaxing the tension upon the ring of the suspensory ligament attached at 

 one border to the lens and at the other inserted into the circle of ciliary 

 muscles. When these muscles contract they pull the choroid coat forward 

 and relax the tension on the ligament. The lens then becomes more convex, 

 taking its natural more spherical shape. Rays of light from nearby objects 

 are then brought to a focus on the retina in near vision (Fig. 17.14). When 

 at rest, the eye is adjusted for far vision. The eyeball is always distended by 

 the fluids within it and when the ciliary muscles are relaxed there is a chronic 

 pull on the suspensory ligament. This flattens the lens, and rays reflected 

 from distant objects are brought to a focus on the retina. 



Imperfections in Convex Lenses. There are imperfections or aberrations 

 in biconvex lenses because the rays that penetrate their thin margins meet 

 in different places from those that pass through them near the center. The 

 spherical aberration of the lens of the eye is partially corrected by the curva- 

 ture of the cornea. Cameras usually have lenses with compensating curvatures 

 fastened to the convex lens (Fig. 17.14). 



Chromatic aberration or color error is also characteristic of single lenses. 

 Short wave lengths are bent more strongly than longer ones. Thus rays of blue 



