Effect of Bodies on Light and Heat. 389 



When two minute pencils of light are admitted through apertures very 

 near to each other, the screen on which the blended pencils fall is streaked 

 with lines absolutely dark. This is interference. In passing around 

 corners some parts of the ray need to travel greater distances than other 

 parts. When such difference amounts to half a wave length the crest 

 of one wave falls in the hollow of another, and they neutralize and pro- 

 duce darkness. Various experiments illustrate this principle, as light 

 admitted through gratings, &c. 



Interference takes place in all wave movements when the crest of one 

 wave falls upon the hollow of another. This is true of the waves of 

 water, of sound, or of light. Whenever there is interference there is 

 stillness, which in the case of the luminous vibrations means darkness. 

 It has been stated that the vibrations of luminous waves are in all az- 

 imuths. Waves, therefore, occupy such relationship to each other that 

 the front of some waves is presented to the edges of others. The front 

 of the wave is in the direction in which its crest and hollow are alter- 

 nately formed ; the direction of its azimuth and of its activity. If a 

 ray strike a mirror edgewise of the ray it cannot be reflected, for it has 

 no activity or elasticity in the edgewise direction of the vibrations. 

 Waves cannot interfere, therefore, when the front of one series is pre- 

 sented to the edge of another. Interference takes place only between 

 vibrations which are in the same azimuth, or while they are in the same 

 azimuth. Waves are naturally of different lengths representing the dif- 

 ferent pitch or color. If two waves of unequal length undulating in the ' 

 same azimuth are traveling the same path, they must interfere at those 

 points in which the crest of one overtakes the hollow of the other. So 

 if the lengths of two waves are to each other as six to seven, at every 7th 

 undulation of the shorter and every 6th undulation of the longer one they 

 will interfere. As stated above, all light is retarded in passing from a 

 rare to a dense medium. Now, if by any means a ray of such retarded 

 light be made to mingle with a ray of the same color or pitch vibrating 

 in the same azimuth, they will interfere, provided the retardation of the 

 one has amounted to a half wave length at the point where they mingle. 

 When such interference takes place there is darkness as to that particu- 

 lar color while other colors remain visible. This sort of interference 

 happens when the two unequally retarded rays passing through a doubly 

 refracting crystal are brought together again. ( See on Polarization in 

 this chap. ) 



The colors of striated surfaces are due to interference that destroys a 

 part of the colored rays leaving the others to be reflected. This happens 

 in the case of fine scratches on glass or burnished metal. The same 

 conditions are realized when mother of pearl is cut and polished across 

 the edges of the very fine layers of which the shell is composed. An 



