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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 develops on Exposure to a Polarised Ray. By J. F. 

 W. Herschel, Esq. F.R.S. Lond. and Edin. Read December 23, 

 1819. [Phil. Trans. 1820, p. 45.] 



When Malus published his discovery of the polarization of light, 

 the list of doubly-refracting crystals was small ; and as the most re- 

 markable of them possessed only one axis of double refraction, it was 

 presumed that the law discovered by Huyghens, applicable to that 

 one, might hold good in all ; but the discovery of crystals with two 

 axes of double refraction has proved the fallacy of such generali- 

 zation, and rendered new and extensive investigations necessary. 



There are two modes of conducting observations on double re- 

 fraction and polarization ; the one turns upon immediate observation 

 of the angular deviation of the extraordinary pencil, the other de- 

 pends upon the separation of a polarized ray into complementary 

 portions by the action of a crystallized lamina. After noticing the 

 advantages of the latter, Mr. H. observes, that to render observations 

 on the tints developed by polarized light available, they must be 

 comparable to each other ; hence the importance of discovering the 

 existence and tracing the laws of those causes which operate to dis- 

 turb their regularity. In the author's first inquiries on the polari- 

 zation of light, he was struck by the great deviation from the suc- 

 cession of colours in their laminae, as observed by Newton, which 

 many crystals exhibit when cut into plates perpendicular to one of 

 then: axes ; and finding this phenomenon unconnected with irregu- 

 larities in their thickness or polish, and uniformly repeated in diffe- 

 rent and perfect specimens, he was led to inquire into their causes, 

 especially as they appeared to form an unanswerable objection to 

 M. Biot's theory, which perfectly explains the tints in crystals with 

 one axis. 



In the several sections of this communication, Mr. H. first enters 

 into a detailed description of the phenomena themselves, which are 

 reducible to one general fact ; namely, that the axes of double re- 

 fraction differ hi their position in the same crystal for the different 

 coloured rays of the spectrum, being dispersed in one plane over an 

 angle more or less considerable according to the nature of the sub- 

 stance. In many bodies, the magnitude of this dispersion of the 

 axes is comparatively trifling ; while in some, not otherwise remark- 

 able for a high ordinary or extraordinary dispersive power, it is enor- 

 mous, and renders all computation of the tints in which it is not 

 taken into consideration completely erroneous. A new element is 

 thus developed, which for the future must enter into all rigorous 

 formulae of double refraction ; and another striking instance is pre- 

 sented of the inherent distinction between the different coloured 

 molecules of light. At the same time, says the author, by the com- 

 plete explanation this principle affords of all the more perplexing 



