284 STUDY OF THE DOUBLE SELEXATES OF THE SERIES E2M(SeO,)o,6HoO. 
former increased 3° 10' for 60° of rise, and that the angle of the latter was hut 
remarkaldy slightly affected hy change of temjoerature, being increased by only 25' 
for 60° of rise. Even in this magnesium group, therefore, in which the caesium salt 
is al3normal, the character of 2 :)rogression which has been found to obtain throughout 
tlie double suljDhate series, and in the zinc group of double selenates akeady studied, 
is still found to hold good, namely, a decided increase of the optic axial angle in the 
case of the potassium salt, a decided decrease in the case of the caesium salt, and a 
com]3arative indifference to change of temperature in the case of the rubidium salt. 
It is somewhat remarkalde that four cases of large dispersion in crossed axial 
jdanes have l^een observed in the course of the author’s work on the simple and 
doidole sulphates, namely, the rhomhic sulphate of rubidium and selenate of caesium 
and the monoclinic double sulphate and double selenate coiitaining caesium and 
inagnesiiim. In each case the phenomenon has been rendered possible by the con¬ 
currence of two conditions, namely, extremely small double refraction (closeness of 
the a and y indices of refraction) and the approximation to identity of the inter¬ 
mediate index of refraction /3 to either the a or y index. The latter condition 
appears to he necessary for crossing of the axial planes to Ije possible, and the 
former condition for magnitude of separation of the optic axes in the two j^erpendi- 
cidarly crossed planes. These two conditions are adequate to render the substance 
highly sensitive to slight differences in dispersion (that is, differences between 
I^G-Lu ya-Li), a dispersion difference of O'OOOS having been shown to he ample to 
cause reversal of the relations of the two neaily identical indices. Such substances 
are bound also to be highly sensitive to change of temjDerature, whicii usuallv 
provokes minute but influential changes in dispersion as well as producing different 
amounts of change of refraction along each of the three princijDal axes of tlie 
optical indicatrix. 
As the occurrence of these conditions has been general for all the four cases of 
crossed axial plane dispersion studied in detail b}^ the author, it woidd appear 
probable that they afford a general explanation of this interesting j^henomenon. 
In the next communication the iron group of double selenates will be described. 
