Page:Proceedings of the Royal Society of London Vol 1.djvu/510

Rh results with those elements which M. Mechain obtained by Laplace’s method, with those obtained by Legendre himself, and with those ultimately deduced as corrected elements from the latest observations, how near an approximation is obtained by the method here given; so that the apparent errors seem rather to be those of observation; which, in fact, are not susceptible of great accuracy even with the best instruments, and with the greatest care, on account of the haze or coma with which these bodies are generally surrounded.

The present experiments, to which the author has been led by dis. covering the singular property of agate described in his former com- munication to the Society, have been attended with results which he considers so extraordinary, that they appear to lead to the very mysteries of physical optics, and exhibit, he says, a series of appear- ances, which far surpass in splendour and variety all other phenomena of light.

This paper treats, ﬁrst, of the polarizing power of the agate; secondly, on that structure of the agate on which its properties depend: thirdly, on peculiar colours exhibited by it; fonrthly, on the depolarization of light; and ﬁfthly, on certain elliptic coloured rings produced by obliquely depolarizing crystals.

With respect to the polarizing power of the agate, Dr. Brewster has before shown that a ray of light transmitted through a slice of laminated agate, cut at right angles to its laminae, may be transmitted through a prism of Iceland spar Without being subdivided, being refracted ordinarily in one direction, and extraordinarily when the principal section of the spar is transverse to the laminae of the agate. The author observed at that time a nebulous light that accompanied the bright image of a luminous object seen through the agate, consisting of rays that were not similarly affected. He now adds. that this nebulous light is omositely affected, being refracted like the extraordinary rays transmitted through Iceland spar, and accordingly disappearing when the bright image is most discernible, and vice versd. But though the polarization of these rays be different, the refraction of both is the same.

In order to convey, as accurately as may be, an idea of the structure of the agate having these properties, the author assists his description by delineations of the‘appearances which the substance itself presents in consequence of, the variations in ﬁneness of the laminae, their curvatures, or opacity. Some of the laminae are white, others transparent; some straight, others variously curved; and where ﬁnest and most transparent, exhibiting an appearance of small waves on a surface of water rippled by a gentle breeze, and depending on small variations of the inclination of the laminae.