Joty—On the Pseudo-opacity of Anatase. | 479 
_ It is thus seen that transmission, save near the basal edge of 
the pyramid, is very limited, almost nil. It will be evident that 
the transmission of the ordinary ray (anatase being optically 
negative, and the ordinary index the greater) will be more 
restrained than the extraordinary. 
The difficulty of passing a ray through these crystals vanishes 
when they are immersed in a substance of high refractive index, 
such as Canada balsam (1°593). The index from balsam to ana- 
tase is 1°584, and the critical angle 39° 9’. Rays entering from 
balsam at an incidence less than 6° 43’ are totally reflected and 
returned. There is thus a wide are of transmission from grazing 
incidence (90°) to 6° 43’, an are of 82°17’. This are is set out in 
the figure. In consequence, light entering from the air, refracted 
by the glass and balsam of the cell in which the crystal is mounted, 
reaching the crystal incident within this arc, passing through and 
again bent both on emergence from the anatase and the containing 
balsam and glass, finally reaches the eye, deviated some 43° from 
its original path. This angle was observed by a refractometer. 
The light coming through the crystal, imbedded in Canada 
balsam, exhibits spectrum colours which, however, appear to be 
pure only at either end of the visible spectrum. That is, when 
the crystal is held at the suitable angle, and illuminated by a small 
and bright source of white light, the red, first brought into view, is 
seen to have the richness and purity of the lower red of a pure 
spectrum. The orange, yellow, and green which succeed are evi- 
dently whitish and impure, but the blue and violet are rich and 
brilliant. This is explained as follows :— 
The ray of white light entering the crystal is resolved into two 
plane polarized rays. One, the ordinary, executes vibrations, the 
refractive indices of which are, for the B wave-lengths, 2°51118, for 
the H wave-lengths, 2°64967. These indices are constant, é.e. are 
independent of the inclination of the rays traversing the crystal. 
The second, the extraordinary ray, vibrating in the plane of inci- 
dence and of the optic axis, has indices which vary with the incli- 
nation of the rays within the crystal; that is, with their direction 
in the ellipsoid of elasticity. When the rays are transmitted at 
right angles to the optic axis, the B vibrations have the index 
2°47596, the H 2°58062.! The substance is, in fact, optically 
1Schrauf, Wien. Ak. Ber, 42, 1860, p. 107. 
