COLOR 163 



given out are always slower than those taken in. A solution 

 of chlorophyll, the green coloring matter of plants, is green by 

 transmitted light, but the light reflected from it has a reddish 

 hue. Kerosene is also strongly fluorescent. The greenish 

 color often reflected from the eyes of animals at night is prob- 

 ably also due to fluorescence. The X-rays, or Roentgen rays, 

 used for locating broken bones, bullets, and the like in the 

 human body, are not visible to the eye, but owing to the fact 

 that they excite fluorescence in various substances, we are 

 able to construct a fluoroscope by means of which the shadows 

 of the bones and other dense objects may be studied. 



137. Complementary Colors. Not only may white light 

 be broken up into the seven primary colors, but we can pro- 

 duce white light by a proper mixture of these colors. If any 

 of the seven colors be missing, however, the light will not be 

 white. Since any of the primary colors may be produced by 

 the proper mixture of red, green and violet-blue, these latter 

 are often regarded as the real primary colors. The color which 

 must be added to another color to make white light is called 

 its complementary color. Among sets of complementary colors 

 are red and blue-green, yellow and blue-indigo, greenish-yellow 

 and violet, and orange and light blue. A colored object is 

 always made more conspicuous by being near its comple- 

 mentary color. It is to be noted that when pigments are 

 mixed, they do not always give results in accordance with the 

 statement just made, for the reason that pigments are not 

 colors but substances which reflect colors. Yellow and blue 

 rays give white light, but yellow and blue pigments give 

 green color. Colors differ considerably in their carrying power, 

 and this does not depend entirely upon their wave length. 

 Yellow is visible from the greatest distance. As the twilight 

 deepens, blue flowers are the first to become indistinguishable, 

 then follow the red and pink, but the yellow are usually 

 visible except on the darkest nights. 



