LIGHT AND VISION 



155 



iar instance of chemical change caused by light is the decom- 

 position of matter on the photographic plate when the picture 

 is made. The most important use of light to the world is in 

 supplying the energy needed by all plant life, and (since ani- 

 mals are entirely dependent on plants) of all animal life as 

 well. The green cells of plants turn the light energy into 

 electric energy and by its aid combine the materials absorbed 

 from the soil and air into food. Plants are the only living 

 things that can thus form food from chemical elements. 



132. Phosphorescence. A large number of substances are 

 now known which have the power of storing up light when 

 exposed to the sun's rays, and of giving it off in darkness. Such 



FIG. 57. Apparent size of square affected by direction of the ruling. 



substances are said to be phosphorescent. In all cases the wave 

 lengths of the light given out are longer than those taken in. 

 Some substances may be made phosphorescent by friction, 

 hammering, or splitting, and others are affected by electricity, 

 heating, etc. Various plants give out rays of light in the dark, 

 and a large number of the lower animals also have this power. 

 The fireflies and glowworms are good illustrations. The phos- 

 phorescence which often crests the waves in the warmer parts 

 of the globe is caused by minute one-celled animals. Lumi- 

 nous paint is now made from substances which are strongly 

 phosphorescent. This paint absorbs light during the day and 

 emits it at night, and is therefore of value for covering match- 

 boxes and other objects which need to be located in the dark. 



