XII. On the polarisation of light by oblique transmission through all bodies, whether crystallized or uncrystallized. By David Brewster, LL.D. F. R. S. Edin. and F. S. A. Edin. In a letter addressed to Taylor Combe, Esq. Sec. R. S

Abstract

Sir, In a letter on “ the Affections of Light in its passage through crystallized Bodies," which I had the honour of transmitting a few days ago to the Royal Society through Sir HUMPHRY DAVY, I alluded to a series of experiments which I had in view for the purpose of generalising the various phenomena which had been described. At the very commencement of this enquiry I have been led to the important general result “ that “ light transmitted obliquely through all transparent bodies, “ whether crystallized or uncrystallized, suffers polarisation " like one of the pencils formed by doubly refracting crystals," and I hasten to communicate to you a brief sketch of the nature and consequences of this discovery. In examining if any change was produced upon common light during its passage along the oblique depolarising axis of mica, I observed, in one position of the mineral, some appearances which indicated a partial polarisation of the incident rays. Upon turning the mica round, so as to preserve its obliquity to the incident pencil, the same phenomena presented themselves in every part of the revolution of the mica, and the quantity of polarised light was found to increase with the obliquity of its incidence. I then substituted a plate of glass instead of the mica, and a similar result was obtained, though the quantity of polarised light was considerably less titan in the first experiment. By adding one plate of glass after another, the number of polarised rays was increased by the addition of each plate, and when the plates amounted to fifteen , the transmitted pencil was wholly polarised at an angle of about 70° 17', and possessed all the properties of that species of light.