Joty—On the Pseudo-opacity of Anatase. A477 
angle of incidence very nearly as great as this, some may escape, 
but the greater part is retained. It will then return to the illumi- 
nated side, and either escape or be again reflected wholly or in 
part. 
It is easy to show that, if a prism of any material be cut to such 
a refracting angle, that this is equal to 2 for the particular sub- 
stance and wave-length, then no light of this wave-length can pass 
through the prism, or, what is practically the same thing, only that 
ray can pass which enters and emerges at grazing incidence. In fact, 
a ray in suck a prism enters at grazing incidence with a refracting 
angle », and preserves the symmetrical position of minimum 
deviation, emerging at the grazing angle. Now the critical angle 
for the mean refractive index of anatase 1s, as we have seen, 
23° 20’, and the opposite faces of the octahedron meet at the apex 
of the octahedron at the angle 43° 24’. This is so nearly the value 
of 2% (46° 40’), that rays refracted according to the mean index, 
even if entering at grazing incidence (90°), are to a great extent 
returned by internal reflection, and those arriving at an incidence 
of less than 60° are totally reflected within the crystal, and that 
not only at the first reflection, but are repeatedly retained from 
escape by internal reflection, total or partial; finally for the most 
part escaping on the side of entry. 
Only near the basal edge of the octahedron are the phenomena 
different. Here rays can pass through the opposite parallel faces 
of the octrahedron, and accordingly a transparent belt should here 
be detected. Unmounted crystals examined for this belt some- 
times exhibit it in the form of a chink of blue light: its visibility 
is, however, often greatly reduced by deep-cut striations near the 
basal edge, which divide and scatter the light both at entrance, at 
exit, and within. 
The accompanying figure shows the path (dotted) of a ray 
entering a vertical section of the octahedron (at the point ‘a’) at 
grazing incidence (90°); also one entering at an angle of incidence 
(60°) such that it is totally reflected at the opposite face of the 
erystal. The course of the last is then traced (full line) till it 
finally is, for the most part, returned (if not previously absorbed) at 
the side of entry. A second ray entering at an incidence of 60°, 
but so near the base (point B) as to meet on second reflection 
the adjoining face above the base, is also traced (dot and dash 
line). 
