245 
or tlie Absorption of Common Light 
2. On the Dlchroism qf Augiie. 
Out of a piece of yellowish-brown augite, without any cry- 
stalline form, for which I was indebted to Robert Ferguson, 
Esq. of Raith, I cut plates with parallel and well polished sur- 
faces. When one of these plates was exposed vertically to com- 
mon light, the transmitted light 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 obli- 
quity increased, notwithstanding the increased thickness of the 
mineral through which the light had to make its way. Upon 
examining the transmitted 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, but still in. the plane of the same neutral 
axis, the intensity of the light gradually diminished, till, on the 
other side of the perpendicular, the plate became absolutely 
impervious to the strong rays of the sun. Upon again examin- 
ing the transmitted light with a prism of calcareous- spar, be- 
fore the plate had become opaque, the pencil which had former- 
ly vanished now re-appeared, and gradually increased in inten- 
sity, becoming more and more green^ while the other pencil, 
which became fainter, grew more and more red, till at a very 
great obliquity, the one pencil became perfectly green^ and the 
other deep blood-red. By exposing the plate to the polarised 
light of the sun, the red and green were alternately absorbed, 
according to the position of the neutral axis, with respect to 
the plane of primitive polarisation. 
If we now take two plates of Augite, one of which has been 
cut from the other, and adjust each of them separately in a po- 
sition where the swi's rays are very much e7ifeebled ; if they are 
then brought together, without altering the inclination of the inci- 
dent light, the sun’s rays wiU penetrate through both the cry- 
stals, even though the one is turned round before the other, the 
incidence remaining the same. If, on the contrary, we ad- 
just each of the plates separately, in a position where the trans- 
mitted light is a maximum, and where the eye cannot endure 
the strength of the solar ray ; and if they are then brought 
together, so that the planes of incidence are transverse to one 
