28 Dr. Brewster on the absorption of polarised light, c ifc. 
already mentioned, but there is also reason to believe, that 
polarised light suffers the same kind of absorption in those 
crystals whose two images have the same tint, and even in 
those which are perfectly colourless. 
I have the honour to be, &c. 
Edinburgh, Oct. \"jth, 1818. 
DAVID BREWSTER. 
To the Right Hon. Sir Joseph Banks, Bart. G. C. B. &c. Sec. Sec. 
