X. On the communication of the structure of doubly refracting crystals to glass, muriate of soda, fluor spar, and other substances, by mechanical compression and dilatation. 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, Notwithstanding the numerous discoveries which have lately been made relative to the polarisation of light, and the optical phenomena of crystallized bodies, not a single step has yet been made towards the solution of the great problem of double refraction. What is the mechanical condition of crystals that form two images and polarise them in different planes; and what are the mechanical changes which must be induced on uncrystallized bodies in order to communicate to them these remarkable properties, are questions which are as difficult to be answered at the present moment, as they were in the days of Huyghens and Newton. In the frequent attempts which I have made to obtain a solution of these difficulties, the polarisation of light by oblique refraction was the only phenomenon that seemed to connect itself with the inquiry; but the hopes of success which this fact inspired, were soon found to be delusive, and the subject resumed its former impregnable aspect. A new train of experiments, however, has enabled me not only to give a satisfactory answer to the questions which have been stated, but to communicate to glass, and many other substances, by the mere pressure of the hand, all the properties of the different classes of doubly refracting crystals. The method of producing these effects, and the consequences to which it leads, will be briefly explained in the following letter.