478 Intelligence and Miscellaneous Articles. 
But it also possesses in a high degree the property of reflecting 
two oppositely polarized beams ; and the great size of the crystals in 
which it may readily be obtained renders it peculiarly fitted for 
optical examination. If one of these crystals be viewed by reflected 
light while it is held with its principal axis lying in the plane of in- 
cidence and reflexion, the reflected light is found to be not pure 
white, but to have a purpleshade. Examined witha rhombohedron 
or an achromatized prism of Iceland spar, having its principal axis 
in the plane of incidence and reflexion, the ordinary image is white 
as usual, while the extraordinary is of a fine purple colour, the phe- 
nomenon having the greatest distinctness when the light is incident 
at the angle of maximum polarization. 
The experiment may be varied and the purple light beautifully 
seen without the use of a doubly reflecting prism, by allowing only 
light polarized perpendicularly to the plane of incidence to fall on 
the crystal; in this case the surface of the crystal appears rich deep 
purple, no white light reaching the eye. 
This property is not possessed by all the planes of the crystal, but 
is limited to the principal prism and brachy- and macrodiagonal end 
planes, in other words, to the planes parallel with the principal axis 
of the crystal. The brachydiagonal doma and OP planes do not 
possess it. Nor is it exhibited by the first-mentioned planes when 
the crystal is turned with its prismatic axis at right angles to the 
plane of incidence. 
All specimens of picrate of manganese do not possess this pro- 
perty to an equal extent. The crystals vary considerably in colour, 
and those which are full red exhibit it more strongly than the amber- 
coloured. Picric acid boiled with aqueous solution of cyanhydro- 
ferric acid and saturated with carbonate of manganese, gives crystals 
of a rich deep colour, which exhibit the purple polarized beam par- 
ticularly well. 
These properties are not possessed by the manganese salt alone, 
but also by the picrates of potash and ammonia ~ (especially when 
crystallized by very slow spontaneous evaporation in prisms of suffi- 
cient size), and the picrates of cadmium and peroxide of iron—with 
this difference, however, that while the prismatic axis of the crystal 
in the case of the cadmium and manganese salts must be in the plane 
of incidence, in the alkaline salts it must be perpendicular to that 
plane. As they all crystallize in the right-rhombic system, it is pro- 
bable that either the alkaline salts on the one hand, or the manganese 
and cadmium on the other, are prismatically elongated in the direction 
of a secondary axis. 
It is convenient that distinct phenomena should have distinct 
names; and none appears to have been assigned to this. Brewster 
speaks of it as a ‘‘ property of light,” and Haidinger uses the word 
“Schiller” for it. The terms dichroism, trichroism, and pleiochroism 
are limited to properties of transmitted light. I therefore suggest 
for the phenomenon here in question the name catachroism, using the 
preposition cara in the same sense as in the word xarorzpigw, to 
reflect (as a polished surface), applying it to express the property of 
