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When Malus published his discovery of the polarization of light the list of doubly-refracting crystals was small; and as the most remarkable 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 generalization, and rendered new and extensive investigations necessary. There are two modes of conducting observations on double refraction and polarization; the one turns upon immediate observation of the angular deviation of the extraordinary pencil, the other depends 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 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 their laminæ, as observed by Newton, which many crystals exhibit when cut into plates perpendicular to one of their axes; and finding this phenomenon unconnected with irregularities in their thickness or polish, and uniformly repeated in different 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.

abstracts of the papers printed in the philosophical transactions of the royal society of london

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 polarized ray