wherry: crystal form and optical properties 



281 



sources of variation render the method of study here proposed of 

 little value in connection with the more complex compounds, it 

 seems likely to be useful for simple ones. 



Certain features of the tetragonal system may now be briefly 

 reviewed. The classes it includes, with their symmetry and typ- 

 ical representatives, are listed in table 2. 



TABLE 2 

 Tetragonal System 



Classes 4, 6, and probably 7 rotate the plane of polarized light, 

 indicating the presence of some asymmetrical arrangement of 

 atoms, and their refraction and axial ratios are therefore likely 

 to show poor agreement. In class 5 the positions of the hori- 

 zontal axes are fixed by the symmetry, so good agreement between 

 the two ratios is possible in the crystals belonging to it. In 

 classes 1, 2, and 3, however, there is a choice between the two 

 sets of horizontal axes, and there is no fundamental reason why 

 the closeness of packing in the layers, which is apparently what 

 determines the refractive effect exhibited all the way around a 

 uniaxial crystal, should be greater in one of these directions than 



