Page:Proceedings of the Royal Society of London Vol 1.djvu/385

Rh person, it was afterwards tried in one who was subject to calculus. consishing of the triple phosphate of magnesia. Though his stomach did not admit the use of stronger acids, the carbonic acid proved highly grateful; and by examination of his urine, it appeared that the phosphates, which before were voided as a sediment of white sand, were now passed only in a state of complete solution, by means of the redundant acid.

{{hi}Supplement to the First and Second Part of the Paper of Experiments for Investigating the Cause of Coloured Concentric Rings between Object-glasses, and other Appearances of a similar Nature. By William Herschel, LL.D. F.R.S. Read March 15, 1810. [Phil. Trans. 1810, p. 149.]}}

The Supplement now oﬂ'ered to the Society, is intended to clear up certain points which have been represented to the author as oh- scure or doubtflﬂ in his former communications, and at the same time to connect more intimately the prismatic experiments of the second paper with those made upon convex glasses, and described in the author' s ﬁrst paper on the subject.

Since Dr. Herschel has heard the originality of his observation of the red bow called in question, upon the ground that a red bow had been observed by Sir Isaac Newton, which is merely the converse of the blue how (the change of colour being dependent upon the di- rection in which the light is received upon the prism), Dr. Herschel ﬁrst endeavours to answer the objection, and reminds us that in his former observations the angular breadth and elevation of the two bows are different; but those of the Newtonian blue and red bows are said to be, and are, necessarily equal. In the Newtonian expe- riment also, the same beam of light is made to exhibit both pheno- mena, being received upon two right-angled prisms, applied base to base, so that one portion of the light is reﬂected upwards, as a blue bow from the under surface of the ﬁrst prism; and the remainder, by hansmission, through the second prism, appears as a red bow to an eye beneath. But in Dr. Herschel’s experiment, the same prism is made to exhibit, to an eye in the same situation, the red bow as well as the blue, by means of light transmitted in an opposite di- rection through the under surface of the prism, without any occasion for a second prism, which (as Dr. Herschel observes) is necessary in the Newtonian method of conducting the experiment.

The next objection replied to by Dr. Herschel, regards the streaks that may be seen adjacent to the bows when a second surface is applied to that side of a prism at which a critical separation of the colours takes place. It has been said that streaks parallel to the bows, though not dependent on critical separation, will in that situation be seen most easily and most distinctly, because the visual ray, under those circumstances, passes with the greatest obliquity between the surfaces.

To this objection Dr. Herschel replies, that these streaks not only