On the Polarization 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. Read January 27, 1814. [Phil. Trans. 

 1814, p. 219.] 



In examining what changes were produced upon light transmitted 

 through mica in the direction of that line which Dr. Brewster calls 

 its oblique depolarizing axis, he observed some appearances indicating 

 a partial polarization : but upon turning the mica round, so as to 

 preserve the same obliquity of incidence, this effect was found not 

 to depend on the position of the axis ; but to be greater or less, in 

 proportion to the obliquity of incidence alone, and to be produced 

 even by a plate of glass substituted for the mica, though not in so 

 great a degree. By transmitting the same pencil of light successively 

 through fifteen plates of glass, at an angle of about 70, the whole 

 of that which is transmitted is polarized ; so that its transmission 

 through agate, its reflection from polished surfaces at a specific angle, 

 or the kind of refraction it undergoes in its transmission through Ice- 

 land spar, depend upon the relative position of the planes of refrac- 

 tion. If a second series of similar plates be presented to light thus 

 polarized, it will also be totally transmitted if the plates be parallel 

 to the former, but totally reflected if, with the same inclination, the 

 planes of refraction be at right angles to each other. 



By experiments made on the number of plates requisite for causing 

 complete polarization at different angles of incidence, the number 

 varied as the co-tangent of incidence. 



The author next endeavoured to ascertain the difference that would 

 be occasioned by using plates of greater refractive density ; and he 

 found that a less angle of incidence was then sufficient for effecting 

 complete polarization by the same number of plates : but the subjects 

 of his experiments were not sufficiently different in refractive power 

 for him to determine with precision their proportional effects. 



Dr. Brewster observes, that the polarization effected by such a 

 series of plates may be employed with advantage in examining those 

 coloured rings produced by topaz, described in his .former communi- 

 cation. 



In consequence of the reflections that take place at each of the 

 surfaces, the principal image seen through a number of such plates 

 is always surrounded with a great number of faint images ; and when 

 the inclination is very considerable, a nebulous image appears that 

 is oppositely polarized, and has the same relation to the bright image 

 as the author had before observed in agate. 



When the coloured rings produced by topaz are viewed through a 

 number of plates so inclined, the two halves of the rings appear com- 

 pletely different ; the colours of one set being complementary to those 

 of the other. 



When the angle of incidence is 54 35' (the angle at which Malus 

 observed reflected light to be completely polarized), then the number 



