24 Dr. Brewster on the absorption of polarised light 
4. Epidote. The cross formed at the resultant axes of epi- 
dote has its diverging branches brown and sap green, and 
they are distinctly seen in common as well as in polarised 
light. The green shades into a pale greenish white as the 
ray recedes from P and P' to C and D, where it is no longer 
divisible into two differently coloured pencils. At O the two 
pencils are brown and green, and at A and B they are brown 
and a paler green. The dichroism of the epidote is distinctly 
marked in common light. Along the axis of the prism, and 
through two of its parallel faces, its colour is a deep orange, 
while through the other two parallel faces it is a yellowish 
green. 
5. Mica. A specimen of this mineral exhibits the branches 
diverging from P and P' exactly in the same manner, whether 
it is seen by common light, or exposed to a polarised ray with 
the plane of the resultant axes perpendicular to the plane of 
primitive polarisation. In these positions the branches PA, 
PB, &c. are a dark brown, and PC, PO, &c. a brownish 
white. When the plane of the resultant axes is in the plane 
of primitive polarisation, the colours of the branches are in- 
terchanged. The dichroism of mica is finely exhibited in 
some of its small and perfect crystals. In some of these its 
colour by common light is greenish yellow along the axis 
of the prism, and of a deep garnet colour when the light is 
transmitted through its faces. 
6 . Anhydrite. This mineral exhibits its dichroism by com- 
mon light. When the light is transmitted in a direction 
parallel to the laminae, its colour is pink ; but when it is in- 
cident in a direction perpendicular to the laminae, its colour 
is a palish yellow slightly tinged with pink. 
