Bowen — Birefringence of Torbernite. 



197 



exact results are given graphically in fig. 1. The wave- 

 length for which there is no difference in the index of the 

 two rays was checked— it was, indeed, fixed with some- 

 what greater accuracy— by examining the interference 

 of prismatic sections. As the drum of the monochro- 

 mator is turned and light of various colors passes 

 through the sections, they remain brightly illuminated 

 through the red and yellow; then rather abruptly in the 

 green, at 515^/x, they become dark and finally as the drum 

 is turned farther they brighten up in the blue, in this case 

 not so abruptly. It will be noted that the dispersion of 

 the ordinary ray is greater than that of the extraor- 

 dinary. 



By reference to the ordinary diagram used to illustrate 







Fig. 2. 









A 







^ 



I / 





/ / 







c* ^ 



V / 





f / 







<> 



V 





// 



/<\ 



/A 





A 





/ / 



OJ 



i ^ 



» 



' \ 



/ 







\ ^ 



\ y 



\ 



A 





« 



\* 



X 



.y 



/OO 200 300 



J^-j" 



Fig. 2. — Showing the manner in which colors combine to give "interfer- 

 ence color ' ' for various values of retardation. 



the interference color of thin sections, an explanation of 

 the abnormal colors of torbernite is readily obtained. 

 Such a diagram is given in fig. 2, where the wave-lengths 

 for three colors are plotted and the color of a section is 

 determined by noting the proportion in which the colors 

 combine for any given value of the retardation. Thus, 

 with a retardation of about 25(W, the violet, green and 

 red are combined in about the proportion in which they 

 are contained in the original light source and, therefore, 

 white light is reproduced. The diagram is based on the 

 assumption that the retardation is the same for all colors, 

 which is never strictly true and in extreme cases, such 

 as the present, may be far from the truth. For torber- 

 nite the retardation is always zero in the green and is 

 very small for adjacent colors. An approximation to 

 the interference color of torbernite sections may be 



Am. Jour. Sci. — Fourth Series, Vol. XLVIII, No. 285.— September, 1919. 

 14 



