Dr Brewster on the Phenomena of Dichrolsm 
confined to this mineral the property of giving two colours. The 
Count de Bournon {Catalogue^ p. 121.) had observed the double 
colour in certain small crystals of mica, and the Marquis de Dr& 
had noticed a similar fact in the tourmaline. During the experi- 
ments to which I have alluded, I found that dichrolsm was a 
very common property of crystallised bodies : that it was relat- 
ed to the axes of double refraction, whether the crystal had one 
or more axes ; and that it arose from the absorption of common 
light, modified by the doubly refracting forces of the crystal. 
As the phenomena of dichrolsm are very beautiful, and can 
be seen without any apparatus, and merely by exposing the 
crystals to common light, I shall describe the most important 
facts as they are seen in Mlca^ Auglte^ and lolite. 
1. On the Dichrolsm (^'particular Crystals Mica. 
There are many crystals of mica which exhibit the phenome- 
na of dichroism, but it is only in some of the small hexahedral 
crystals, which are transparent in a direction perpendicular to 
the lamina, that they are seen to the greatest perfection. 
In one of these crystals, where the inclination of the result- 
ant axes was about 11°, I found that it was highly transparent 
in a direction coincident with the plane of the laminae, even at 9 , 
thickness of Jth of an inch ; in this position the extraordinary ray 
only was transmitted. As the inclination of the ray to the laminae 
increased, the intensity of the transmitted light diminished; 
and in a direction perpendicular to the laminae, the crystal 
was perfectly opaque. A candle, whose light was freely trans-^ 
mitted through a thickness of 0.243 of an inch across the 
faces of the hexagonal prism, was completely invisible through 
the terminal planes, when the thickness wiis only 0.040 of 
an inch. Another crystal which, in one direction, was as 
transparent as the ordinary specimens of olivine^ would not ad- 
mit, through a thickness of ^^^th of an inch, a single ray of the 
meridian sun on the 20th of May, w4ien it passed along the 
axis of the prism. The ordinary ray, which was entirely lost 
in one direction, became gradually visible in thin plates, and at 
last of equal intensity to the other ray, as the ordinary light 
ibnned a p;reatcr angle witli the laminae. 
