93 
by doubly refracting crystals. 
related to its axes of doable refraction. Dichroite has two 
axes of extraordinary refraction, the two resultant axes being 
inclined 62° go' to each other, or 31 0 25' to the axis of the 
prism. If we cut a plate of dichroite with four parallel faces, 
each of which is perpendicular to the resultant axes, and is 
inclined 31 0 25' to the axis of the prism, and expose it either 
to common or to polarised light, so as to have the plane of 
its resultant axes perpendicular to the plane of primitive 
polarisation, we shall observe the branches of blue and white 
light diverging in a beautiful manner from its poles P, P\ 
(See the figure in p. 17, which is a very imperfect representa- 
tion of the phenomenon.) The white light becomes more blue 
from P and P' to O, and more yellow from P and P' to C and 
D. When the plane of the resultant axes is in the plane of 
primitive polarisation, the poles P, P' are marked by spots of 
white light, but every where else the tint is a deep blue. In 
the plane CADB, the mineral when seen by common light 
is yellow mixed with a small quantity of blue, polarised in 
an opposite plane. From A and B towards P and P 7 , the 
yellow image becomes fainter till it changes into blue, and 
the weak blue image is reinforced by other blue rays till the 
intensity of the two blue images is nearly equal. The faint 
blue image increases in intensity as the incident ray ap- 
proaches from C and D to P and P'. From P and P' to O 
one of the images is whitish, and the other deep blue, but the 
whiteness gradually diminishes towards O, where they are 
both equally blue.* 
* Two crystals of Dichroite which I directed to be cut so as to exhibit these phe- 
nomena, are in the cabinet of Thomas Allan, Esq. to whose friendship I have been 
indebted for several of the minerals noticed in this paper. 
