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IV. On the action of crystallized bodies on homogeneous light , 
and on the causes of the deviation from Newton's scale in the 
tints which many of them develope on exposure to a polarised 
ray. By J. F. W. Herschel, Esq. F. R. S. Bond, and Edin. 
Read December 23, 1819. 
Since the period of the brilliant discovery of Malus of the 
polarisation of light by reflection, the investigation of the 
general laws which regulate the action of crystallized bodies 
on light, has advanced with a rapidity truly astonishing, and 
the labours of an Arago, a Brewster, and a Biot, have al- 
ready gone far towards completing the edifice of which that 
distinguished philosopher laid the foundation. When Malus 
wrote, the list of doubly refracting crystals was small, and 
the most remarkable among them possessing only one axis of 
double refraction, it seems to have been for some time, tacitly 
at least, presumed that the law discovered by Huygens, and 
since re-established in the most rigorous manner for that one,* 
might hold good in all. The discovery, by Dr. Brewster, 
of crystals possessing two axes of double refraction, or two 
* The author of the article on Polarisation, in the 63d Number of the Edin- 
burgh Review, just published, is guilty of a most unpardonable mistake, in as- 
serting, (p. 188), as deducible from Dr. Brewster’s experiments, that the Huy- 
genian law is incorrect, for carbonate of lime. Dr. Brewster’s general formulae for 
crystals with two axes resolve themselves into the Huygenian law when the axes 
coincide, of which case it is only an extension. That excellent philosopher, if I 
understand English, in the paragraph which gave rise to this strange assertion, only 
means to declare his opinion that it remains undemonstrated. 
