142 Proceedings of Philosophical Societies. [Feb. 



Article VIII. 



Proceedings of Philosophical Societies. 



ROYAL SOCIETY. 



Jan. 13, 1820.— Mr. Herschell's paper " On the Action of 

 Crystallized Bodies on Homogeneous Light, and on the Causes 

 of the Deviation from Newton's Scale ot Tints which many of 

 them develope on Exposure to a polarized Ray " was concluded. 

 When Malus published on the present subject, the number of 

 doubly refracting crystals known to philosophers was very- 

 limited ; and as the most remarkable of these possessed only one 

 axis of double refraction, it was presumed that Huygen's law 

 applicable to that one might hold good in all. But the discovery 

 of crystals with two axes of doulole refraction has shown the 

 fallacy of this generalization, and rendered new investigations 

 necessary. The author proceeded to observe that there are two 

 modes of conducting observations on double refraction and pola- 

 rization; the one is founded on the immediate observation of the 

 angular deviation of the extraordinary pencil ; the other 

 depends upon the separation of a polarized ray into comple- 

 mentary portions by the action of crystallized laminae. The 

 author preferred the latter method, and after pointing out its 

 advantages, observed, that to render observations on the tints 

 developed by polarized hght available, they must be capable of 

 being compared with one another; hence the importance of 

 knowing the existence, and tracing the laws of those causes 

 which operate to disturb their regularity. In the author's first 

 inquiries on the polarization of light, he was struck by the great 

 deviation from the succession of colours in thin laminas, as 

 observed by Newton, that many crystals exhibited when cut 

 into plates perpendicular lo one of their axes ; and finding this 

 phenomenon unconntcted with irregularities in their thickness 

 or polish, and uniformly repeated in different and perfect speci- 

 mens, 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 ciystals with one axis. 



In the several sections of this elaborate paper, the author 

 entered into a detailed description of the phenomena, which are 

 reducible to one general fact, viz. that the axes of double refrac- 

 tion differ in their position in the same crystal for the differently 

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

 an angle more or less considerable according to the nature of the 

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

 the axes is comparatively small, v/hile in others not remarkable 

 for a high ordinary or extraordinary dispersive power, it is very 

 great, and renders all computations of the tints in which it is not 

 taken into account completely erroneous. A new element is 

 thus developed, which the author observed must in future enter 



