394 Dynamic Theory. 



A film of a crystal of gypsum, like Iceland spar, reduces the azimuths 

 of vibration to two which are at right angles to each other. If a pair 

 of nicol prisms be mounted in front of an electric lamp, and their axes 

 crossed at right angles, no light passes. If a gypsum film be intro- 

 duced between them in such position that one of its planes of vibration 

 is parallel with that of the first nicol the polarizer, its other plane of 

 vibration will be parallel with the plane of the other nicol the an- 

 alyzer, and in that position the light is still cut off. But if the gypsum 

 film be turned 45 in azimuth, its two planes of vibration will be oblique 

 to both the polarizer and the analyzer, and the light will pass. That is, 

 the light is polarized in the first nicol, the polarized beam is divided 

 into two in passing through the doubly refracting gypsum film, and 

 these two half beams get back together again in passing through the 

 second nicol, the analyzer. But by the law of double refraction, in 

 passing through the gypsum one of the half beams is retarded more 

 than the other, so that when the two are brought together again in the 

 analyzer, they are found to be vibrating in the same azimuth, but one a 

 little behind the other. These are conditions of interference, and con- 

 sequently the waves producing some of the colors in the white light are 

 quenched, leaving the others to be transmitted. Chromatic effects of 

 this sort must follow in all cases where plates of doubly refracting crys- 

 tals of proper thickness are used. If monochromatic light, as red for 

 example, instead of white light be used, and the crystalline plate be 

 shaved to a wedge-shape, thinner on one edge than the other, the in- 

 terferences can abolish only one color, the red, and a system of bands 

 alternately red and black, is transmitted. If the crystalline plate be 

 circular and of varying thickness, increasing from the center outwards, 

 the alternate bands of color and darkness take the form of rings, these 

 phenomena arise from the unequal retardation of the light waves in the 

 crystal, thus shortening some of the wave lengths more than others. 

 Whenever this difference in the shortening amounts to any multiple of 

 'half-wave lengths they interfere and are reduced to darkness. When 

 it amounts to whole lengths or their multiples the colors are sustained. 

 When white light is used it is of course the same as using lights of all 

 the colors at once. And so the effect is to form a series of colored 

 rings and dark interference rings for each of the colors contained in the 

 white. But these various effects are so superposed on each other that 

 the dark rings of one are covered more or less by colored light of others. 

 Any solid double refracting substance whatever, whether mineral or or- 

 ganic, when it is made thin and is sufficiently transparent to allow the 

 passage of polarized light, will produce chromatic effects of one kind or 

 another when the separated rays are rejoined by the analyzer, because 

 such structures produce double refraction on account of the quasi crys- 



