12 Dr. Brewster on the absorption of polarised light 
shall first describe the phenomena which are presented by 
crystals with One axis, and then explain the modifications 
which these phenomena undergo when the number of axes 
is increased. 
Sect, I. On the absorption of polarised light by Crystals with 
One axis of double refraction. 
If we fasten upon one side of a rhomboid of colourless calca- 
reous spar, a circular aperture of such a magnitude that the 
two images of it appear distinctly separated when viewed 
through the spar, we shall find, by exposing it perpendicu- 
larly to common light, that the two images are perfectly 
colourless, and of the same intensity in every position of the 
rhomboid. Hence if O be the quantity of transmitted light, 
we shall have the ordinary image 0 = -§-Q, and the extra- 
ordinary image E == \ Q. 
When the rhomboid is exposed to polarised light, the in- 
tensities of the images vary with the azimuthal angle (a) 
which the axis of the rhomboid forms with the plane of pri- 
mitive polarisation, and may be represented by the formulae 
O = Q cos. 2 a ; E= Qsin. 2 a. But since O cos. 2 a -f- Q sin. 2 a = Q 
we have O + E = Q; that is, the sum of the intensities of 
the two pencils is in every position equal to the whole trans- 
mitted light, and therefore the rays which leave any one of 
the images by a change of azimuth, are neither reflected 
nor absorbed, but pass over into the other image . The or- 
dinary phenomena of double refraction, consequently, afford 
us no reason for conjecturing that the crystals which possess 
this property absorb the incident light in any other way than 
is done by all other bodies, whether solid or fluid. 
