British Association for the Advancement of Science. 101 



Prof. Whewell communicated a letter from T. G. Bmit, Esq., 

 of Bristol, showing the progress made in the Tide-calculations 

 which had been assigned to the former. Elaborate and delicate 

 observations on the tides have for some time been carried on at 

 this place, but the details are too extensive for insertion here. 



On the use of Mica in polarizing light, by Professor Forbes. 

 The author explained the method of preparing mica used by him 

 since 1836 for the polarization of heat and light. The mica is 

 exposed for a short time to an intense heat in an open fire, by 

 which the laminas are so subdivided, that a pellicle of extreme 

 thinness contains a sufficient number of reflecting surfaces, to 

 polarize very completely the light or heat transmitted through it 

 at a certain degree of obliquity. Being struck by the resem- 

 blance to metallic lustre, which the mica thus acquires, he also ex- 

 amined in 1836 some of its leading properties with regard to light, 

 and found 1st, that the light reflected from a plate of mica so 

 prepared (which light is very intense,) is but feebly polarized in 

 the plane of incidence ; and 2d, that the reflection so far resembles 

 that at metallic surfaces, that when plane-polarized light is reflec- 

 ted from it, the plane of reflection being inclined to that of prim- 

 itive polarization, the light is found to be elliptically polarized. — 

 Prof Lloyd observed, that, simple as this discovery might appear 

 to those not conversant with this intricate subject, yet he consid- 

 ered it as highly important, not only as furnishing a method of 

 polarizing light elliptically and circularly, more simple than any 

 previously in use, but as it would also essentially aid the re- 

 searches of those engaged in perfecting the theory of this inter- 

 esting branch of physical optics ; indeed, did time permit, he 

 thought he could show that it furnished a clue at least to the so- 

 lution of some of the difficulties which had hitherto opposed the 

 progress of inquiry. In polarizing light elliptically or circularly, 

 it was well known that the condition to be obtained is, that two 

 rays should encounter one another in different phases, or, to speak 

 the conventional language, of which one is accelerated by a half 

 or some proportional part of a wave, while the other is either sim- 

 ilarly or dissimilarly retarded. In this case, this condition is ob- 

 tained, and with respect to a greater quantity of rays than in other 

 processes, for those rays which, after being reflected at the first 

 surface of those very thin laminae of mica which they met, were 

 afterwards encountered by those rays which passed into the 



