THE RAINBOW 161 



will be brought together again, and the rays produced 

 will be white, as they were before refraction. So we can 

 prove, both by breaking up white sunlight and by putting 

 its colored parts together, that it is composed of many 

 rays of different colors. The colored band produced by 

 the single prism is called the solar (i. e., the sun's) spectrum. 

 It was first studied with great 

 care by Isaac Newton, in 

 1672. 



FIG. 156. 



i on TO. T> * 1~ T'U Reversed Prism. Sunlight that has been 



lOO. IRC KainDOW. Ine broken up can be recombined to 



finest solar spectrum we see 



in Nature is the rainbow. In order to see a rainbow we 

 must look at falling rain, and the sun must be behind us, 

 and 42 degrees, or less, from the horizon. Half the dis- 

 tance from the zenith (c/. 177) to the horizon is 45 de- 

 grees. On a small scale a rainbow may often be seen in the 

 spray of a waterfall or of a lawn sprinkler. In Nature two 

 bows are often seen together: a primary one, red on the 

 outside of the arch and violet on the inside, and a secon- 

 dary one, outside the primary one, and with the colors of 

 the primary rainbow in reversed order. 



In forming a rainbow each drop of water acts both as a lens and as 

 a mirror. It refracts the sun's rays as they enter the drop, reflects 

 them from side to side within the drop, and then refracts them as they 

 re-enter the air. 



From the outside of the rainbow arch only red rays reach our eyes. 

 The drops inside the arch send to us, in order, orange, yellow, green, 

 blue, and indigo rays. The lowest drops send the violet rays. 



A halo, or ring of light, around the moon or the sun is probably due 

 to a similar bending of light rays by the thin, icy clouds of the upper 

 part of the atmosphere. 



