98 INTRODUCTION TO GENERAL SCIENCE 



71. COLOR 



It is customary to speak of the color of objects, as if color 

 existed. There is really no such a thing as color, except as a 

 sensation. Colors are not due to materials, as such, but to 

 the fact that some materials are capable of absorbing part of 

 the light and reflecting the rest. White light is made up of 

 all the colors of the rainbow. (See next section.) Accord- 

 ingly, if we look at some material which absorbs all colors 

 except red, it will appear red. This is because it reflects the 

 red light, or, more scientifically, it reflects those particular 

 wave lengths which give to our eye the sensation of red. The 

 same explanation applies to all other colors. If, on the other 

 hand, we use some source of light which does not produce 

 pure white light, objects do not appear the same by it as by 

 sunlight. This is because some of the waves which the object 

 can reflect are not present, and therefore cannot be reflected. 



The color of the sky is due to the fact that the light is par- 

 tially blocked by small particles of dust in the air; those par- 

 ticular wave lengths which gives us the sensation of blue are 

 blocked most, and are thus either stopped or reflected. Ex- 

 cept in the neighborhood of the sun, the light which illumi- 

 nates the sky is due to light which has struck the earth and has 

 been reflected up to the sky. Most of the light which has 

 been reflected from the earth passes on into space, but those 

 very short wave lengths which give us the sensation of blue 

 are reflected back by the particles of the air, and we see the 

 sky as blue. The fact is, the sky is absolutely black ; that is, 

 it has no color whatsoever. In those countries where there 

 are no manufactories, and where the air is extraordinarily clear, 

 the sky is very dark blue; and balloonists tell us that the 



