THE PHENOMENA OF LIGHT 343 



those of the rainbow the so-called solar spectrum. The 

 screen or wall serves simply to intercept and throw the 

 " colors" back into the eyes of observers. These colors 

 were in the sunlight that passed through the prism, and they 

 were changed from their straight line direction as they passed 

 into the glass and again still further as they emerged from 

 it. This change in direction of the light is caused by change 

 in density of medium. The color elements contained in 

 sunlight are bent unequally, and they diverge more and more 

 the farther the screen is away from the prism. This makes 

 the spectrum longer and longer without completely sepa- 

 rating the colors, or preventing their blending into one 

 another. 



If now in the path of the emergent ray a blue glass plate 

 is held, it is the blue light of the spectrum only (or largely) 

 that gets through the plate. The other colors are almost 

 wholly absorbed by the coloring material in the glass. If 

 a piece of common window glass is used all the colors pass 

 through it, while with red glass nearly all the colors but red 

 are stopped by the glass. When both blue and red plates 

 are put in the emergent beam together, none of the colors 

 appear on the screen. Each of the plates has absorbed the 

 colored light which the other let pass. By choice of the 

 proper pigments to go into glass, and by varying their pro- 

 portions, beautiful color effects indoors may be obtained 

 with windows by combining in varied patterns glass of 

 different colors. We say that glass is blue or red or any 

 other color according to the kind (color) of the light it 

 transmits. 



When only the blue light of the spectrum appears on the 

 screen, and a sheet of paper (or any white object) is held in 

 its path, the paper in a darkened room appears blue. But if 

 a red screen be used instead of a white one, it looks black 

 as result of absorption of the blue light. There is then lack 



