365 



person, it was afterwards tried in one who was subject to calculus, 

 consisting 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. 



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 offered to the Society, is intended to clear 

 up certain points which have been represented to the author as ob- 

 scure or doubtful 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 bow (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 transmission, 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, \vhich (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 ap- 

 plied to that side of a prism at which a critical separation of the co- 

 lours 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 



