Page:Proceedings of the Royal Society of London Vol 1.djvu/513

Rh of plates requisite for complete polarization of the transmitted beam is 30 : and since, under these circumstances, the whole of the light that is not reﬂected at the ﬁrst surface is transmitted through the whole series, the author observes, that transmission is not in this case a maximum at a perpendicular incidence, and that the law employed by Bouguer fails by reason of these newly-discovered properties of light, of which that distinguished philosopher was not aware.

The celebrated discovery of Malus, of the polarization of light by oblique reﬂection, and its connexion with the properties of doubly- refracting crystals, is perhaps the most important discovery that has been made in optics since that of the principle of the achromatic telescope; but the author observes, that it does not furnish us with any information of the manner in which these crystals effect polari- zation, and that the present discovery of polarization by oblique re- fraction supplies the connecting link between these two classes of facts, and holds out a prospect of a direct explanation of the leading phenomena of double refraction.

Should the present paper meet with the approbation of the Society, Dr. Brewster promises a further communication of experiments on the polarization of light by reﬂection, in which he designs to show that the law observed by Malus is not general, and that the principle has been completely overlooked by him; as it depends on the pro- portion which the quantity of light reﬂected bears to that which is transmitted when incident at the polarizing angle. When light is incident upon water at the polarizing angle, he remarks that only Tﬁv is reﬂected; that even from glass only 1—HT is reﬂected; but when realgar, diamond, or chromate of lead are employed, then at the polarizing angle these bodies reﬂect as much as one half of the light, and consequently have not power to polarize all that they re- ﬂect.

The experiments detailed in the present letter were conducted exactly in the same manner as those detailed by Capt. Kater in his former communication, for the purpose of comparing a new Cassegrainian telescope, made by Mr. Crickmore of Ipswich, with the Gregorian used in the former experiments. The diameter of the large speculum in this instrument is 4‘9 inches, but was reduced by a ring of pasteboard to 3'6, in order to render the illumination equal to that of the Gregorian, in which the large speculum measured 3'95 inches.

The areas exposed to the light being estimated at 7152 and 10593, and the magnifying powers at the same time being 157 and