Page:Proceedings of the Royal Society of London Vol 1.djvu/511

Rh In this communication the author again notices certain coloured images, seen on each side of the ﬂame of a candle, or other luminous object seen through the agate, one at 10?, and a second at 21", but which he is not yet able to explain, and supposes to be a new case of production of colours.

Dr. Brewster next gives the result of experiments on the transmission of light previously polarized, through various substances, and notices those positions of crystallized bodies in which the polarization continues unchanged, and those intermediate positions at which complete depolarization takes place; and adds, that such effects are also occasioned by plates of horn, gum-ambic, glue, tortoise—shell, and even plate glass.

In addition to these properties, which mica, topaz, and rock crystd possess in common with other crystallized bodies, Dr. Brewster ob- serves, that they have the power of depolarizing in certain oblique positions, which he considers peculiar to them. And at the same time these bodies have certain oblique positions in which they do not de- polarize, and which he terms neutral.

In the preceding experiments depolarization has been effected by the interposition of a second body, through which the rays are trans- mitted after having been previously modiﬁed by some polarizing sub- stance; but the author observes that these effects may both be pro- duced by the same crystals, if the direction of the light be such, that after reﬂection from the posterior surface it will coincide with the oblique depolarizing axis.

It was in attending to the affections of light thus polarized and depolarized by a plate of topaz ﬁlu-th of an inch in thickness, that the author observed certain elliptical coloured rings, which he con- siders entirely new; and as he thinks them important, he takes much pains to describe their various dimensions and successions of colours, and represents them in coloured drawings.

When a doubly-refractng substance is employed to view these rings, the two images seen of them are diﬁ'erently coloured, the co- lours of one set being complementary to those of the other.

When a plate of agate, or a plane reﬂector at a speciﬁc angle of inclination are employed, then only one or the other set is seen, ac- cording to the relative position of the planes of incidence; and it is in the instance of using the plane reﬂector, that these rings appear with such peculiar brilliancy on account of the absence of all foreign light, which can, in this mode of making the experiment, be com- pletely avoided. _

In addition to the above experiments, of which the author gives a detailed account, he remarks, that light reﬂected at a particular angle from the surface of blue steel is polarized, and thence infers that the oxide is a thin transparent ﬁlm; that light is partially polarized by reﬂection from all metallic surfaces.

That light from white clouds or blue sky is partially polarized, but that no part of the moon's light has suffered any degree of polarization.