chap, xxx.] INTERFERENCE OF LIGHT. 407 



when the first Nicol and the plate have their 

 principal planes at an angle of 45 to one another. 

 If with this position of the plate the second Nicol be 

 rotated till the two Nicols stand at an angle of 45 to 

 one another, the colour disappears and the light 

 becomes white. When the Nicols are parallel, 

 another colour is produced complementary to the 

 former. Thus, with the plate of selenite in the first 

 position described, the Nicols being crossed the colour 

 is red, and with the Nicols parallel the colour is 

 green. The colours are due to what is called 

 interference. Suppose two waves on the surface of 

 water, if the crest of one coincides with the crest of 

 the other, the height of the united wave will be 

 doubled. In such a case both vibrations would be in 

 the same phase, the vibrations of each wave would be 

 proceeding in the same direction, and would be in the 

 same position at the same time. Suppose, however, 

 the crest of one wave coincided with the hollow of 

 another wave, then a particle which would be at its 

 extreme displacement above the line of rest for the 

 crest of one wave would be at its extreme displace- 

 ment below its line of rest for the hollow of the other 

 wave. That is to say, the waves being similar, the 

 particle would be at the same moment under the 

 influence of two equal and opposite forces, and would, 

 therefore, remain at rest. This is the phenomenon of 

 interference. If, however, the crest of one wave did 

 not absolutely coincide with the hollow of another, 

 then the particle having received an impulse to 

 vibrate in one direction, would have already started 

 in that direction before it received the impulse in the 

 opposite direction. Its motion would still be inter- 

 fered with but not completely arrested. The distance 

 between the crest and the hollow of a wave is a half 

 wave length. Thus, we see that when two waves 

 differ by half a wave length, they extinguish one 



