Joty—On the Pseudo-opacity of Anatase. 481 
The readiest mode of exhibiting the very perfect transparency 
of many of these black and seemingly opaque crystals is to imbed 
them in melted sulphur or fused silver nitrate. The first men- 
tioned substance is excellent, as formerly observed. The results 
may be examined either when the mountant is melted or solidified. 
Through the yellow translucent sulphur we see, in the latter case, 
the sky-blue anatase, like certain clear, pale sapphires; every 
enclosure and flaw within being clearly visible. Some of them 
have large cavities; others enclosures having the form of spirally 
arranged plates (Brookite ?), or quite irregular cloudy enclosures. 
Nevertheless, examined by the naked eye in the air, no trace of 
transparency may be revealed. 
A similar phenomenon is, in a less degree, exhibited by other 
substances of high refractive indices. A cleavage fragment of 
blende will present a perfect opacity on edges which, when more 
carefully examined, are found to be transparent. It is remarkable 
_ that anatase crystallises with such an apical, interfacial angle as to 
exhibit the phenomenon nearly at its greatest degree of perfection. 
Even if rigorously possessing 2¢ as the value of the refracting 
angle, but little would be gained in opacity, while the capabilities 
of direct cross-transmission from diagonally opposite and parallel 
faces of the octahedron would be increased. The nearly complete 
transparency of wiserine is a consequence of the subordination of 
the acute octahedral faces in its case. 
It will be gathered from the foregoing explanation of the 
behaviour of anatase that the high lustre and smooth surface play 
an essential part in bringing about its pseudo-opacity. Any dull- 
ness or roughness permits light both to enter and leave the crystal 
irregularly, and the phenomenon described does not occur. 
