XXI. On the structure of the crystalline lens in fishes and quadrupeds, as ascertained by its action on polarised light. By David Brewster, LL. D. F. R. S. Lond. and Edin. In a Letter addressed to the Right Hon. Sir Joseph Banks, Bart. G. C. B. P. R. S
Dear Sir, There is, perhaps, no subject in natural history which has excited so much attention, as the structure and functions of the eyes of animals; and there is certainly none which has so ill repaid the anxiety and labour with which it has been investigated. The physiologist was naturally let to study the mechanism of an organ through which man receives the noblest of all his enjoyments; and the natural philosopher, considering it as the work of infinite intelligence, ardently anticipated the improvement of optical instruments from the imitation of this perfect model. It is discouraging however, to estimate the real amount of the labours of the one, and to perceive how little advantage has been derived from them by the other. The most prominent functions of the eye are still very imperfectly understood, and the improvement of the telescope has been retarded, rather than advanced, by the pursuit of a false analogy. I have, therefore, some satisfaction in being able to throw additional light upon a subject of such difficult investigation, and so generally interesting both from its optical and physiological relations. Having found that the doubly refracting structure could be communicated to glass and other bodies, by giving them a variable density either through the agency of heat or mechanical pressure, I was led to conclude, that the same structure would be found in the crystalline lens of fishes and other animals, which was known, by direct experiment, to increase in density towards the centre. I had formerly examined the action of the crystalline upon polarised light, without obtaining any new result; but I now placed such reliance on the truth of the general principle, that I resumed the subject with the utmost confidence of success.