or THE DOUBLE SELENATES OF THE SERIES R2.M(Se04)2,6H20. 
271 
neighbourhood of wave-length 560 the a vibrations occur parallel to the symmetry 
axis b, and the vibrations take place in the symmetry plane. 
Ratio of Axes of Optical Ellipsoid .—These ratios are as follows :— 
Axes of optical indicatrix : 
a : 13 :y = O’OOOO : 1 : UOOSS. 
Axes of optical velocity ellipsoid : 
a : h : C = 1-0001 : 1 : 0-0963. 
Molecular Optical Constants .—The calculated values of these constants are as 
under :— 
Axis of optical indicatrix. 
a. 
13. 
y- 
Specific refraction, , = n . . ^ 
|C 
0-1026 
0-1026 
0-1035 
^ (vd + 2) d 
1 tl 
0-1052 
0-1052 
0-1062 
, r 1 1 • '/d - 1 M i 
Molecular refraction, ,, • , = ni . i 
n~ -(-2d 1 
c 
70-06 
70-08 
70-72 
^ G 
71-84 
71-85 
72-51 
Specific dispersion, ito — He 
. 
0-0026 
0-0026 
0-0027 
Molecular dispersion, nta — nu • • 
1-78 
1-77 
1-79 
Molecular refraction, 7 ... 
(v 
c 
119-64 
119-69 
120-99 
Optic Axial Angle. —Three capital section-plates of ' 
this salt were obtained 
grinding, out of large ljut very jjerfect crystals, perpendicular to the first median line. 
Such sections require to be very thick, 5 millims. or more, in order to exhibit clear 
interference tigures, on account of the extraordinarily small amount of double refrac¬ 
tion. Sections perpendicular to the second median line show no axial brushes even in 
bromnaphthalene, the obtuse angle of the optic axes being too laig’e for measurement. 
The plan has accordingly been adopted which was employed in the case of caesium 
magnesium sulphate, for the determination of the true optic axial angle, namely, 
measurement while the sections perpendicular to the first median line were immersed in 
a liquid whose refractive index was identical with the mean index of the crystal. Cedar 
oil answers the purpose admirably, its refractive index for sodium light being 1-520. 
The optic axial interference tigures afforded by thick section-plates are both 
exceptional and very beautiful. In wliite light a deeply coloured figure is presented, 
in which the hyperbolic brushes are broad spectra. In monochromatic light a rapidly 
changing series of figures is presented, commencing with the brushes separated by 
30° for red light, and approximating closer and closer to each other through the 
yellow and green, until for F-light the separation is only 13°. Passing into the blue, 
the hyperbolic brushes eventually coalesce, for light of wave-length 466, into the 
uniaxial rectangular cross, and the lemniscates become circles. The exact wave-length 
was readily determined by taking the circle reading of the monochromatic illuminator 
corresponding to tlie production of the uniaxial figure, and ascertaining the wave- 
