I 



wherry: crystal form and optical properties 327 



SUMMARY 



In this paper the study of the "refraction ratios" of crystalHzed 

 organic compounds is suggested, these ratios being obtained by 

 substituting in the standard refractivity formula the refractive 

 indices exhibited in different directions. It is pointed out that 

 these ratios as well as the crystallographic axial ratios are con- 

 nected with the spacing of the planes of atoms in the space- 

 lattices of the substances, and that comparison of the two ratios 

 may be expected to throw considerable light on the type of space- 

 lattice represented in each case. Such a comparison is made for 

 the substances urea, penta-erythrite, mellite, and three double 

 propionates, and their refraction ratios prove to be the exact 

 inverse of the crystallographic axial ratios. The relations in 

 several other substances are also discussed, and deviations from 

 inverse proportionality are shown to be connected with atomic 

 anisotropism, asymmetry of carbon atoms, etc. Further work 

 on other substances is planned. .yi^XrT> 



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