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 sufliciently 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 dilferent ; 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
