OPTICAL PROPERTIES "I RY8TAL9 is;* 



nation until the rel'raeted r.-iy is exactly 1/2 a wave length 

 behind the reflected ray, or 1/2 X, then darkness will be the 

 ivMilt. From this point the illumination will increase until the 

 refracted ray is a whole \v:ive length behind the reflected ray, when 

 there will l>e a maximum illumination. There will therefore he 

 kinds of light, representing a maximum light at each whole wave 

 length that one ray is retarded behind the other, as at 1, 2, 3, and 

 there will be a band of minimum illumination at points, as 1/2, 

 3/2, 5/2, at which one ray is retarded an odd number of 1/2 wave 

 lengths behind the other. This condition of alternate bands of 

 light and darkness obtains only when monochromatic light is used ; 

 when white light is used, which is composed of waves of all lengths 

 or colors, and which differ in their velocities in passing through 

 the wedge, their dark and light areas on the wedge will not corre- 

 spond, and the area which will be dark for yellow will be light for 

 red. with a result that the surface viewed with reflected light will 

 show color bands (these bands may be seen on the quartz wedge 

 when held at the proper angle). Beginning at the thin edge of 

 the wedge, all the interference colors will have appeared in 

 order, when the retardation has reached one wave length, or X; 

 then they are repeated in the same order, when 2 X is reached, 

 and again to 3 X. 



These color effects due to the interference of light are well illus- 

 trated by the play of colors on soap bubbles ; in the iridescent films 

 of carbonates, oxides, or oil on the surface of water ; in the cleav- 

 age fractures of such a clear mineral as calcite, and in the small 

 internal and irregular fractures of the opal. 



Order of colors. In the series of colors caused by the inter- 

 ference of light, those which appear first, on the thin end of the 

 wedge, or are caused by a retardation of one wave length or less, 

 are termed the colors of the first order ; those from X to 2 X, the 

 second order ; and those from 2 X to 3 X, the third order ; etc. Above 

 the fourth and fifth orders the individual colors are not well defined 

 and return to the high order gray. The lower orders of colors are 

 each characteristic in intensity and tone, and with experience may 

 easily be distinguished ; as, for instance, red of the first order, from 

 red of the second or third orders ; since the order of color yielded 

 by sections of approximately the same thickness of the various 

 double refracting minerals is a measure of their double refraction, 

 it is most important that one should be able to recognize the colors 

 of various orders. The most important of these are : 



