270 MR. A. E. TUTTON OX A COMPARATIVE CRVSTALLOGRAPHICAL vSTFDY 
Ptefractive Indices of Ceesium Mao-nesium Selenate. 
O 
Index. direction of Light. Prism 1. Prism 2. Prism 3. Prism 4. Prism 5. Prism 6. Mean, 
vibrations. 
Parallel 2nd 
median line lying 
in symmetry 
plane. 
Parallel symmetry 
axis, It, now 2nd i 
median line. 
Parallel 
symmetry axis, 
In symmetiy 
plane at right 
angles to 1st 
median line. 
f Parallel 
I 1st median line. 
Li 
C 
Xa 
T1 
F 
G 
Li 
C 
Xa 
T1 
F 
G 
Li 
C 
Xa 
T1 
F 
G 
1-5142 
1-5147 
1-5177 
1-5209 
1-5246 
1-5-302 
1-5144 
1-5149 
1-5178 
1-5211 
1-5247 
1-5303 
1-5143 
1-5144 
1-5143 
1-5148 
1-5149 
1-5148 
1-5179 
1-5179 
1-5179 
1-5212 
1-5211 
1-5210 
1-5250 
1-5246 
1-5250 
1-5305 
1 - 5.303 
1-5308 
1-5200 
1-5200 
1-5201 
1-5205 
1-5205 
1-5206 
1-5237 
1-.5235 
1-5237 
1-5270 
1-.5267 
1-5270 
1-5306 
1-5307 
1-5310 
1-5363 
1-5363 
1-5366 
1-5143 
1-5148 
1-5177 
1-5209 
1-5247 
-5145 
-5150 
-5178 
-5210 
-5247 
1-5306 
-5143 
-5148 
-5178 
-5210 
-5248 
1-5-305 1-5-306 1-5-304 
1-5146 
1-5151 
1-5181 
1-5211 
1-5249 
1-5145 
1-5150 
1-5179 
1-5211 
1-5248 
— 1-5-305 
1 - -5201 
1-5206 
1-5236 
1-5269 
1-5-307 
1-5-36-3 
-5201 
-5206 
-5236 
-5269 
-5308 
-5364 
The mean refractive index (mean of all three indices) for sodium light is 1-5198. 
The intermediate index /3, corrected to a vaciinm, is very accurately expressed 
throughout the whole length of the spectrum by the following formula :— 
1 -5033 + 
52 000 000 OOO 
+ - • 
The (( index is given bv diminishing the constant l'o033 bv 0'0002 as fax as C of 
the .S23ectrum, by O'OOOl between C and Tl-light, it is afforded exactly by the formula 
for wave-lengths between that of Tl-light and that of blue light, while for G-light 
O'OOOl requires again to be subtracted from the constant. The y index is afforded 
by the fomnula fairly accurately if the constant is increased by O'OOoZ. 
Alteration of Refraction hy Increase of Temperature .—Measurements were carried 
out at 70° in Na-light with prisms 3 and 5, affording respectively a and y, and a and 
/3. The latter prism exhibited only a single inextinguishable image of the slit at 70°, 
consisting of two identically situated (overlapping) images. It will be subsequently 
shown that at this temperature a uniaxial interference figure is also observed. 
Both j)i’isms afforded values which indicated that the refractive indices are 
reduced by 0'0014 for 55° rise of temperature. For wave-lengths beyond the 
