22 
Dr . Brewster on the absorption of polarised light 
tant axes of the crystal, the blue and the greenish yellow 
light have the form of a cross, the branches of which diverge 
from the two poles of no-polarisation. 
2. Augite. When a plate of yellowish brown augite was 
exposed vertically to common light, the transmitted pencil 
had a moderate intensity ; when it was inclined to one side, in 
the plane of one of its neutral axes, the light became more 
and more intense as the obliquity increased, notwithstanding 
the increase of thickness in the direction of the ray. By exa- 
mining the light with a prism of calcareous spar, it was found 
to be all polarised in a plane perpendicular to the plane of 
inclination. When the plate was now inclined, from this last 
position in the opposite direction, the intensity of the light 
gradually diminished till the plate became absolutely imper- 
vious to the strong rays of the sun. The pencil which had 
formerly vanished when the light was analyzed by Iceland 
spar, now re-appeared, and gradually increased, becoming 
more and more green, while the other pencil, which became 
fainter, grew more and more red, till at a very great obli- 
quity the one pencil became perfectly green, and the other a 
deep blood red. By exposure to strong polarised light, the 
red and the green were alternately absorbed, according to 
the position of the neutral axis with respect to the plane of 
primitive polarisation. 
2. Dichroite or Iolite. This curious mineral has been long 
known to exhibit by common light a deep blue colour along 
the axis of the prism, and & faint yellow or grey colour in a 
direction perpendicular to it. I have ascertained that these 
two colours are those of its ordinary and extraordinary 
images ; and even when seen by common light, that they are 
