S42 Dr Brewster 07Z the Absorption of Polarised Light 
very that philosophers could expect to investigate the general, 
subject with any hopes of success. A series of phenomena of 
this kind presented themselves to the author of this paper when 
he was studying the polarising structure of super-acetate of 
copper, and having been thus led to examine other artificial 
crystals and minerals, he found that the phenomena were much 
more general and important than he had at first supposed. 
If we take a prism of Beryl of a bluish-green colour, and expose 
it to light polarised, either by transmission thrpugh Iceland-spar*, 
or by reflection from glass at an angle of 57% we shall find that 
it transmits only a beautiful blue light when its axis is perpen- 
dicular to the plane of polarisation, and only a greenish-white 
light when the axis coincides with that plane, the transmitted 
light passing from the former to the latter tint, while the cry- 
stal is moving from the first into the second position. Hence it 
is obvious that the green light is absorbed in one position, and 
the blue light in the other position. Now, this absorption varies 
with the angle which the polarised ray forms with the axis of 
the prism, being a maximum when that angle is 90% and va- 
nishing altogether when the ray passes along the axis. 
If we now cut the crystal of beryl into a prism, so as to se- 
parate the two images which it forms by double refraction, we 
shall find that these two images have different colours, the one 
image having the same colour that would have been produced 
by exposing it in one position to polarised light, and the other 
image having the colour that would be obtained by turning it 
round 90°. Here, then, we have two singular properties of 
this class of crystals, which always appear to accompany one 
another, viz. the property which the extraordinary refracting force 
possesses of selecting certain rays out of the compound beam 
of common incident light, and the property of absorbing these 
rays in one position, and the supplementary rays in another po- 
sition, when the incident light has been previously polarised. 
The property which has now been described as belonging to 
Beryl^ I have found in other twelve crystals with One axis. 
The colour of the ordinary and extraordinary images, or of the 
absorbed pencils, is shown in the following table. 
* See this Journaly vol. i. p. 290. and vol. ii. p. 170. 
