The phenomena which the author here endeavours to explain, he says, were first noticed by Prof. Robinson of Edinburgh, and published by Mr. Benjamin Martin in his Treatise on Iceland Crystal. It would not be easy to describe the appearances without reference to the figures that accompany the paper. The peculiar specimens of calcareous spar in which they occur, have been supposed to have fissures, with a disposition to split in the direction of the longer diagonal of the rhomboid. But the author considers it as an interrupting stratum, since it appears to him not to bear the most remote resemblance to a fissure, but resembles a vein or film uniting the two prismatic portions of a rhomboid. Neither are the phenomena produced, when two surfaces artificially polished in the direction of the supposed plane are applied to each other ; but they are produced when a thin film of sulphate of lime is interposed as a vein between two such portions of Iceland spar, and the surfaces are closely united by a cement of strong refractive power.
The supposition of such a crystallized vein, serves also to explain the varieties observable in the characters of the phenomena in different instances, which must vary according to the thickness of the vein, and according to the position of its polarizing axis.
The author also describes certain new instruments for exhibiting complementary colours produced by polarized light; but the descriptions require the aid of his figures to render them intelligible.
Since in the examination of objects so minute and so distant as these satellites, it is necessary not only to magnify for the purpose of removing them from the body of the planet itself, but also to collect as much of their light as may be, in order to render the impression they make on the eye sensible, no instrument less than a 20-feet telescope is sufficient for discerning them; and even with an instrument of this description it is but with a favourable atmosphere, and through a small part of their orbit round the primary, that Dr. Herschel has been able to make his observations ; and he has also occasionally availed himself of the greater light that is to be obtained by concave eye-glasses, notwithstanding the smallness of the field of view, and other objections to their use. The magnifying powers employed on
