MISCELLANEOUS PHENOMENA. 



319 



which in its most general form may be stated as follows : Given 

 a cause which produces an effect which is proportional to it. 

 Then two such causes acting together produce an effect which is 

 the sum of the effects which they would produce if they acted 

 separately, and the total effect may be divided into two parts 

 which correspond to the two parts of the cause, or in other 

 words, each cause produces the same effect that it would produce 

 if it were acting by itself. One of the best examples of this 

 principle is that light passes from two windows, for example, 

 through the same region to the eyes of two observers and each 

 observer sees his window distinctly, that is to say, the light 

 travels through the given region from each window exactly as if 

 it were traveling through the region alone. This principle of 

 superposition is quite accurately true in most of the phenomena 

 of the electromagnetic field. It was discovered, however, by 

 Kerr, in 1875 that an isotropic transparent substance such as 

 glass or oil becomes doubly refracting when subjected to a strong 

 electric field. 



23. The Zeeman effect.* About 1900 it was predicted by 

 Lorenz and experimentally verified by Zeeman, that the light 

 emitted by a hot vapor is altered in a peculiar way when the 

 vapor is placed in an intense magnetic field. The character of 

 this alteration when the emitted light travels parallel to the lines 

 of force of the magnetic field is as follows : Imagine an atom to 

 consist of a positively charged nucleus with one or more nega- 



A B 



Fig. 23. 

 *See Lodge's Electrons, pages 109-115. 



