MONOCLINIC DOUBLE SELENATES OF THE IRON CROUP. 
409 
choice of cedar oil to be made. The prism in question was one formed by two 
excellent faces of p {110}, the pair which form the acute angle 72° 2', and the edge 
between which is often replaced by a strip-face of b {010}. To preserve it fully 
transparent during the rather prolonged measurements with seven wave-lengths of 
light from the spectroscopic. monochromatic illuminator, the two p-faces in question 
were covered with miniature glass plates cemented by quickly drying hard balsam in 
benzene. The edge of such a prism is parallel to the vertical crystal ax is c, and is not 
very far (10° 27') from parallel to the second median line, the a axis of the optical 
ellipsoid, light vibrating parallel to which affords the a index of refraction. The 
index corresponding to rays vibrating parallel to the prism edge would, therefore, 
have a value not far removed from a. The other direction of vibration of rays passing 
through the prism at minimum deviation was that of the symmetry axis b of the 
crystal, and as this is absolutely identical for all wave-lengths with the intermediate 
axis /3 of the optical ellipsoid it furnishes the refractive index /S directly with absolute 
accuracy. The object of this set of measurements was thus to determine /3, the 
image of the slit corresponding to this index being that transmitted when the Nicol 
prism in front of the eyepiece of the goniometer-spectrometer was arranged at 0°; 
the image obtained when the Nicol was at 90° corresponded to vibrations parallel to 
the prism edge and thus afforded a value slightly higher than that of the minimum 
index a. 
A second natural prism was subsequently employed in a similar manner ; it was 
formed by two excellent faces of q {Oil}, those between which also comes the narrow 
line-face b (010) when developed. The prism angle in this case is 52° 12'. The edge 
of such a prism is parallel to the inclined crystal axis a, and only 3° 23' removed from 
the y axis of the ellipsoid, so that vibrations parallel to it (those transmitted with 
the Nicol at 90°) afford almost exactly the y indices. The other rectangular vibration 
direction of the prism, like that of the first prism, was that of the symmetry crystal 
axis b, so that this prism also afforded directly and accurately the intermediate 
refractive index (3, the image corresponding to it being that observed when the Nicol 
was at the 0° position. 
The two values of /3 obtained from these two natural prisms, and their mean value, 
are as follows :— 
Light. 
^-faced prism. 
<jr-faced prism. 
Mean (3. 
Li. 
1-5136 
1-5152 
1-5144 
C. 
1-5141 
1-5158 
1-5149 
Na. 
1-5173 
1-5191 
1-5182 
T1. 
1-5207 
1-5223 
1-5215 
Cd. 
1-5225 
1-5242 
1-5234 
F. 
1-5244 
1-5262 
1-5253 
Gr. 
1-5307 
1-5321 
1-5314 
