274 GENERAL SCIENCE 



made and a sensitive plate is exposed in it back of the 

 pin hole, which acts as a lens. 



A camera is a box, black inside, and made so that 

 light can enter only through a converging lens. The 

 lens, when focused, throws an image on a plate or film 

 which is covered with a chemical that is sensitive to 

 light; each part of the plate is affected according to the 

 amount of light striking it. The light parts of an object 

 which is being photographed throw more light into the 

 camera than the darker parts, and these light parts of 

 the object will affect the sensitive plate more than the 

 darker parts. The exposed plate is developed by washing 

 it in the proper chemicals, and it is then known as the 

 "negative," because its shades are opposite to those of 

 the object photographed. Positive prints are made by 

 allowing light to pass through the negative to sensitive 

 paper, which is then developed by washing it in the 

 proper chemicals. 



190. The Spectrum. When a beam of ordinary white 

 light passes through a prism it is not only refracted but is 



also dispersed, i.e. it is 

 separated into what ap- 

 pears to the eye as a 

 series of different hues 

 or colors, called a spec- 



SPECTRUM FORMED BY A GLASS PRISM trum ' ^ order of the 



colors in such a spec- 

 trum is red, orange, yellow, green, blue, and violet, the 

 red having the least, and the violet the greatest, angle of 

 deviation, as shown in the diagram. 



191. Rainbow. A very beautiful natural spectrum, 

 with which everyone is familiar, is the rainbow. The 

 rainbow is usually seen just after a rain during the latter 



