THE EYE AND THE SENSE OF SIGHT 103 



obliquely from one medium into another of different den- 

 sity, its direction is changed, the ray is bent, or refracted, 

 we may say. In passing from the atmosphere through the 

 prism, the ray of light is twice turned from its original 

 direction. Second, different colors are refracted in differ- 

 ent degrees; red rays, or those having the longest waves, 

 are bent least; the violet rays, those having the shortest 

 waves, most of all; and the colors between vary in this 

 respect in the order of their arrangement in the rainbow. 



137. Images formed by Lenses. A lens is a transparent 

 medium, having at least one curved surface. Rays of light 

 from any point pass- 

 ing through a lens 



are bent either toward 

 each other or more 

 widely apart, accord- 

 ing to the arrange- 

 ment of the surfaces 

 of the lens. When Fig. 60. Diagram of the formation of an 

 they are bent toward hna s e with a lens - 



parh nfhpr thpv mav a an object sending off light. 



ney ] nay b leng c image of the object a 



be brought together 



at a point called the focus. If the focus is allowed to fall 

 upon a screen or other suitable surface, an accurate image 

 of the object from which the rays come is produced 

 (Fig. 60). By using a properly prepared plate the pho- 

 tographer fixes this image and produces a permanent 

 picture. 



138. The Nervous Apparatus for Vision may be briefly said 

 to consist of : (1) the membrane of the eye, called the 

 retina, which receives the end filaments of the optic nerve ; 

 (2) the optic nerve; and (3) the visual center in the brain, 

 which receives the stimulus conveyed by the optic nerve 



