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II. On the Lotus which regulate the Absorption of polarised light 
by Doubly Refracting Crystal s. By David Brewster, LL. D. 
F.R.S. Lond. an Edi In a Letter addressed to the Right 
Hon. Sir Joseph Banks, Bart. G. C. B. 
Read November 12, 1818. 
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 com- 
pound 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 pheno- 
mena which they presented, when cut at different angles with 
the axis, and when exposed in different positions to a polarised 
ray. The ~ ults which I obtained during this investiga- 
tion 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 sub- 
stance 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 
