THE EYE AND THE SENSE OF SIGHT 



113 



the retina (Fig. 68). In the camera a glass lens serves 

 to bring the rays of light to a focus upon the screen. In 



Jig. 68. Formation of an image on the retina. 



the eye the cornea and the crystalline lens accomplish the 

 same object. 



152. Accommodation (Fig. 69). The camera is pro- 

 vided with an apparatus for changing the distance of the 

 lens from the sensitive plate or screen, so that light from 

 objects at different distances may be 

 focused. The eye is likewise supplied by 

 means of what is called accommodation, 

 which is the power of the eye to adjust 

 itself to objects at different distances. 

 This power is due primarily to the varia 

 ble shape of the crystalline lens. In the 

 camera, the lens and the screen which 

 receives the image are moved nearer to- 

 gether or farther apart in order to change 

 the focus. In the eye the same object is 

 gained by changing the convexity of the 

 lens itself. This is accomplished by the 

 contraction and relaxation of the ciliary muscle, which lies 

 just beyond the outer margin of the iris in the front part 

 IOCY'S PHYS. 8 



Fig. 69. Dia- 

 gram show- 

 ing how the 

 lens changes 

 its form. 



