122 LIGHT 



of the eyeball. This retina is made of nerve fibers 

 and endings, through which the image is reported to 

 the brain. 



Near sight is caused by too long an eyeball; the 

 image is formed in front of the retina. Concave glasses 

 may correct this fault; they spread the rays, making 

 them come to a focus farther back. Far sight is due to 

 an eyeball that is too short ; the image is formed behind 

 the retina. Convex glasses will bring the image forward 

 and correct the trouble. 



142. The Photographic Camera. The camera is a box 

 into which no light can enter except through a lens c 



* 3 - This lens is 



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 P just far enough from 



I c ^ a sensitive plate a so 

 e that the rays from an 

 outside object will be 

 brought to a focus on 



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a, forming an image 

 there. The plate is 

 FlG - 96 covered with a sub- 



stance upon which the light acts, affecting each point 

 according to the amount of light focused upon it. Later 

 the plate is developed and fixed in a dark room, after 

 which the image appears plainly. 



143. The Microscope. When an object is placed 

 nearer to a lens than its principal focus, the rays that 

 come from the object will spread apart after passing 

 through the lens. Such rays then entering the eye 

 make the object seem larger, or magnified. 



