344 Dt Brewster on iJie Absorption of Polarised Light 
bhie colour in one direction, and a yellowish-green in the oppo- 
site direction ; and several specimens of idocrase had an orange- 
yellow tint along their axis, and a yellowish-green tint in a di- 
rection perpendicular to their axis. The exhibition of two co- 
lours in the same mineral by common light, was first seen in 
lolite, and the Abbe Haiiy had the boldness to give it the name 
of Dichroite from this property, on the presumption that nature 
had limited it to this mineral. 
In crystals with two axes the preceding phenomena receive 
some curious modifications, for a particular account of which we 
must refer to the paper itself. The tw'o colours which appear 
in the ordinary and extraordinary pencils are now seen diver- 
ging rectangularly from the resultant axes of the crystal ; and 
this effect, when seen in dichroite, constitutes one of the finest 
appearances in optics. The two streams of blue light which is- 
sue from its poles of no-polarisation, are finely contrasted with 
the bluish-white beams which diverge from the same poles 
The following table contains the tints in several crystals, in 
which the effect will be seen in every azimuth. 
List of absorbing Crystals with Two axes. 
Plane of the resultant 
Plane of the resultant axes 
axes in the plane of 
perpendicular to the plane 
prim 
1, polarisation. 
of prim, polarisation. 
Topaz blue 
White 
Blue 
green 
White 
Green 
greyish blue 
Reddish grey 
Blue 
— — — pink 
Pink 
White 
pink yellow 
Pink 
Yellow 
■ yellow 
Yellowish white 
Orange 
Sulphate of barytes 
yellowish 
purple 
Lemon yellow 
Purple 
yellow 
Lemon yellow 
Yellowish white 
— ■ ■ orange 
yellow 
Gamboge yellow 
YeUowish white 
Kyanite 
White 
Blue 
Dichroite 
Blue 
Yellowish white 
Cymophane 
Yellowish white 
Yellowish 
Epidote olive green 
Brown 
Sap green 
whitish gr. 
Pink white 
Yellowish white 
Mica 
Reddish brown 
Reddish white 
* For an account of the method of cutting the Dichroite to see this effect, and 
of the remarkable properties of super-acetate of copper, avgite, mica, See. the reader 
is referred to the original paper. 
3 
