II. On the laws which regulate the absorption of polarised light by doubly refracting crystals. By David Brewster, LL. D. F. R. S. Lond. and Edin. In a letter addressed to the Right Hon. Sir Joseph Banks, Bart. G. C. B

My Dear Sir, In examining the polarising structure of acetate of copper, my attention was drawn to certain changes of colour which this crystal exhibited when exposed in different positions to polarised light. As this variation of colour was independent of the thickness of the plate, and of any analysis of the transmitted pencil, I had no hesitation in regarding it as a new affection of light, and in ascribing the phenomenon to the absorption of the homogeneous tints which formed the compound colour of the crystal. I therefore collected all the natural and artificial crystals which were characterized by any peculiarity of colour, and examined the various phenomena which they presented, when cut at different angles with the axis, and when exposed in different positions to a polarised ray. The results which I obtained during this investigation were singular and unexpected, and I am persuaded will throw considerable light on that property of transparent bodies, by which they detain and assimilate to their own substance a portion of the rays which penetrate them, while the rest are freely and copiously transmitted. As this faculty of absorbing light is related to the axes of double refraction, I 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 ofcolourless calcareous 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 perpendicularly to common light, that the two images are perfectly colourless, and of the same intensity in every position of the rhomboid. Hence if Q be the quantity of transmitted light, we shall have the ordinary image O = ½ Q, and the extraordinary image E = ½ Q.