476 Scientific Proceedings, Royal Dublin Society. 
Some few years ago I had occasion, when exhibiting to an 
audience the trimorphism of titanium oxide, to imbed a couple of 
the lustrous black anatase crystals in Canada balsam. Bright 
flashes of blue, red, and greenish light then showed themselves, 
- emitted near the acute extremities of the octahedron, the slide 
being held considerably below the level of the eye, with the 
crystals in a vertical position, and the light coming from above 
and behind the slide. I put the matter aside till recently, 
ascribing, without much consideration, the brilliant appearance to 
diffraction colours arising from striations on the octahedral faces 
and internal reflections within the slide. Only recently I have 
found that the light is transmitted, being refracted and dispersed 
by passage through the acute pyramids of the octahedron. Hold- 
ing the slide at the proper angle against the light so as to produce 
the brilliant silvery blue light over the lower octahedral face, it 
was found possible, using a lens, to see objects held above and 
behind the slide in close contact with the glass: the meshes of 
wire gauze, for instance. The wires of the mesh are then seen to 
be bordered with spectrum colours, just as large objects appear 
when examined through a glass prism held close to the eye. 
Placed on the stage of the microscope, and an image of a mantle- 
burner formed beneath the crystal by the substage condenser, 
every detail of the luminous meshes could be seen through the 
greater part of the crystal. In this experiment, it could be 
plainly observed that the true colour of the anatase was a pale 
blue. 
Other specimens were examined with the same results, in 
balsam and in melted sulphur (refractive index about 2). In the 
last material (even if it be let grow solid around the anatase), the 
clear transparency and pale blue colour of the crystals may often 
be seen to extend through the entire mass of the crystal. 
To what, then, is the apparent opacity of the lustrous black 
anatase due? An explanation is readily found. 
The refractive indices of anatase are, for D wave-lengths, 
w = 2°03536, « = 249588 (Schrauf), the mean index being given 
as 2'024. The corresponding critical angle in air (@) is 23°20’. A 
ray of light, therefore, entering the octahedron, and meeting the 
opposite face at any angle of incidence greater than this, is totally 
reflected, and retained within the crystal. If it meets it at an 
