278 wherry: crystal form and optical properties 



The atomic weight W being always, and the refractivity M usually 

 the same throughout, the several values of R must be propor- 

 tional to those of the density p, or factors corresponding to it, 

 in the several directions. The ratio of the R's, which may be 

 termed the ''refraction ratio," should therefore give some 

 information as to the structure of the substance. 



Because o^the existence of dispersion of double refraction or 

 change in double refraction with wave length of light, the refrac- 

 tion ratio is not constant throughout the spectrum; but the 

 following list of the approximate dispersions of a few typical 

 substances, obtained mostly by extrapolation from recorded 

 data, indicates that the effect of this phenomenon is ordinarily 

 negligible, the variation in the ratio rarely exceeding the prob- 

 able error of the data, 3 units in the third decimal place, over 

 practically the whole visible spectrum. The relation will not 

 hold, of course, in the ultra-violet, where these substances show 

 absorption bands; but this does not affect the conclusions reached 

 in this paper, for the atoms themselves are anisotropic for these 

 short wave lengths. 



TABLE I 

 Approximate Dispersions op Double Refr\ tion of Selected Organic 



Substances 



UNIAXIAL 



Wave lengths. 



i-Erythrite 



Guanidine carbonate. 

 Urea" 



Calcium formate 



Cane sugar 



Oxalic acid (anhj^d.)" 



400 



1.558 

 1.514 

 1.504 



700 



1.541 

 1.492 

 1.480 



1.526 

 1.551 

 1.465 



1.507 

 1.537 

 1.440 



400 



1.535 

 1 . 505 

 1.625 



700 



1.517 



1.482 

 1.597 



1.599 

 1.582 

 1.650 



1.573 

 1.569 

 1.620 



DOUBLE 

 REFRACTION 



400 



0.023 



0.009 

 0.121 



700 



0.024 

 0.010 

 0.117 



MAXIMUM 



DOUBLE 



REFRACTION 



0.073 

 0.031 

 0.185 



0.066 

 0.032 



REFRACTION RATIO 



400 



1.036:1 

 1,015:1 

 0.837:1 



700 



1.039:1 

 1.018:1 

 0.834:1 



EXTREME 

 REFRACTION RATIO 



0.899:1 

 0.956:1 



0.180 0.758:1 



0.904:1 

 0.954:1 

 0.750:1 



" Extrapolated from new measurements by the writer, made by the immersion 

 method. 



The studies of crystals by X-rays which have been carried on 

 in recent years have shown that it is possible to regard the atoms 

 as lying in definite layers; in simple substances planes pass through 



