220 ELEMENTARY SCIENCE 



whistle; you have seen the flash of a gun before you heard 

 the report; and you have seen the lightning flash before 

 you have heard the thunder. Sound travels at the rate 

 of about eleven hundred feet a second, but light has the 

 tremendous velocity of one hundred and eighty-six thousand 

 miles a second. Yet, as you may have heard, so remote 

 are some of the distant stars that it requires many years 

 for their light to reach us. 



Color. Now light-waves themselves are not all of the 

 same length. What we see as white light is simply the 

 composite effect of light-rays whose wave-lengths are 

 slightly different. It is possible to separate the rays 

 which compose white light in such a manner that each 

 kind of ray produces a separate effect. You have seen 

 this result, and you have often marvelled at the beauty 

 of it. You have seen it whenever you have looked at a 

 rainbow. 



A rainbow gives us what is called the solar spectrum, 

 by which is meant a separation of the visible rays of the 

 sun in such a manner that each kind produces its own effect 

 upon the eye. These different effects we call colors. 

 As you may have learned in using paint or water-colors, 

 white is not a color, but is a "combination of all colors," 

 while black is "the absence of color." 



Now we are in a position to define color. Why is it 

 that one object looks green while another may look red? 

 It is simply because the surface of the green object is of 

 such nature that it absorbs all the rays of light that strike 

 it, except those which produce the effect of green. Similarly, 

 a red object reflects only those which produce an effect 

 of red. So we may define the color of an object as the 

 effect produced on us by the light-rays which it reflects. For 



