280 MR. A. E. TUTTON ON A COMPARATIVE CRYSTALLOGRAPHICAL STUDY 
Fig. 5. 
Axial ratios of optical indicatrix. 
T1 le axial ratios of the optical indicatrix of rubidium magnesium selenate are 
intermediate 1)etween those corres})oiiding to the potassium and csesium magnesium 
salts. The total cliange on passing from one salt to another is much the greater for 
the passage from the rulddium to the Ccesium salt than for the passage from the 
potassium to tlie ru])idium salt. The apparent anomaly as regards the latter change 
in the case of the y ratio has already been fully explained. As regards total change, 
it will also 1)6 observed that the amount is considerably less along the direction 
of the axis y than along the other two axes, along which the change is about equal. 
Tlie most strikiim circumstance about the magnesium series of curves, and in which 
they (lifter remarkably from tliose given iir the author’s last communication on the 
zinc grou|) of double selenates, is the closeness of the a curve to the /S straight line ; 
and that the convergence of the a and y curves, which graphically rejoresents the 
diminution of double refraction with increase of atomic weifflit of the alkali metal, 
residts in consecpience in actual contact (intersection) of the a and ^ curves. It is 
tlius at once made clear that the extraordinary optical properties of cfesium 
magnesium selenate, involving aj)parent uniaxial refraction phenomena and inter¬ 
ference figures in convergent polarised liglit, are the direct results of tlie operation of 
the rule now established, so far as the sei'ies lias yet been studied, that the double 
refraction decreases at an accelerating rate with increase of the atomic weight of the 
alkali metal. The curves are drawn from the values for sodium light at the ordinary 
temperature, and, although on the scale employed contact appears to be just attained, 
this is not really so for sodium light, altliough it requires a high-power eyepiece to 
separate the two images of the slit of the sjiectrometer in Na-light afforded by a 
60°-})rism ground to yield a and It would be true, however, for sodium light at 
the temperature of 78°, and even for litliium light at 94°, while it is alisolutely true 
