Page:Proceedings of the Royal Society of London Vol 1.djvu/524

Rh of 300 or 400 times, and no polishing can remove or modify them. Sometimes 3000 of them may be counted in an inch, while in another part of the same specimen they become much coarser; and a corresponding appearance may be detected in the substances which have received the impression. The axis of extraordinary reﬂection is always perpendicular to the direction of these grooves. Dr. Brewster thinks that the general reﬂection by which the common image is formed, is the effect of the repulsive force of the whole surface, acting at such a distance, that its irregularities do not interfere with the equality of the angles of incidence and reﬂection ; but that which has escaped this reﬂection is subjected to the inﬂuence of the grooves, the form of which, as he shows from the phenomena, must be curvilinear. The spectra do not resemble those which are produced by ordinary inﬂection, and which are observed in circumstances materially different. Dr. Brewster has in vain attempted to obtain coloured impressions from the Labrador spar, and from several of the metallic oxides : he ﬁnds that the crimson and green light of the mother—of—pearl are dependent on its thickness, like the colours which are seen in the common thin plates, but that they appear at much greater thicknesses.

The last peculiarity Which the author has found in this interesting substance, is the manner of its polarizing light. In crystallized bodies, two portions of light are differently polarized with respect to the direction of the plane of incidence; but here the transmitted and reﬂected light are polarized in the same direction. The polarization is the most complete when the angle of incidence is about 60°, and when the thickness is about one fortieth of an inch; the transmitted light is in this case wholly polarized. If the plate is thicker, the transmitted light is wholly polarized at a smaller angle of incidence; and this polarization remains unaltered when the superﬁcial reﬂection is destroyed by the contact of a substance of equal refractive density.

Dr. Brewster concludes with observing, that the subject is far from being exhausted; and that if the investigation could be carried on with the aid of analogous phenomena, we might conﬁdently look forwards to some great change in the fundamental principles of physical optics.

Although the author is most ready to admit the accuracy of Mr. Troughton’s method of dividing, which was described in our Transactions for the year 1809, and by which the mural circle at Greenwich has been divided, it appeared to him that some improvement might be made in regard to simplicity and facility of execution.

The general principle of the method here described by Capt. Kater is. in fact, the same as that of the beam compass; but his apparatus,