280 WHERRY : CRYSTAL FORM AND OPTICAL PROPERTIES 



with tabular habit and basal cleavage are likely to have a positive 

 axial ratio (c greater than a) and those with prismatic habit and 

 cleavage a negative one (c less than a) is also useful. All these 

 features are, however, likely to be connected only with the larger 

 atoms, and the presence of layers of smaller ones to be shown only 

 by the appearance of minor forms, or even not to have any 

 external expression at all. But as all the layers combine in the 

 production of refractive effects, it is essential, before any com- 

 parison can be made, that the true axial ratio, based on all the 

 atomic planes, be ascertained. 



In the few anisotropic inorganic substances thus far studied by 

 means of X-rays the standard axial ratios appear in fact to be 

 produced only by certain of the larger atoms, smaller ones, such 

 as oxygen, failing to find expression in them. But it seems 

 likely that in organic compounds, where the relative sizes of the 

 atoms do not differ so markedly, all might share in the location 

 of the prominent forms, and the true axial ratio be identical with 

 the standard one. This class of substances has therefore been 

 investigated first, their refraction ratios and axial ratios being 

 compared in order to ascertain whether inverse relations really 

 exist; later papers will take up inorganic compounds. And at the 

 start only those that crystallize in the tetragonal system will be 

 considered, since its geometrical relations are the simplest, and 

 will therefore form a good foundation upon which to base future 

 studies in other systems. 



Exact inverse relations can of course not be expected to hold 

 in all cases. For instance, if the atoms in a layer are alternately 

 considerably above and below the central plane, yet not far 

 enough to become close-packed into new layers, the effect of 

 the spacing on the refraction will be modified. Variations of 

 several units in the second decimal place might be attributable to 

 this cause. Further, if any of the constituent atoms are them- 

 selves anisotropic, which is probably the case with those giving 

 rise to intense colors, as well as with those present in asymmetrical 

 combination or position, the value of the refractivity, M, in the 

 formula will vary with the direction, and even the -first decimal 

 place of the refraction ratio may be affected. While these 



