1820.] Royal Society. 297 



In a subsequent part of the paper, Mr. D. described the 

 action of the sulphate of platinum upon jelly, with which it forms 

 a precipitate, and for which, in the author's opinion, it constitutes 

 the best test hitherto known. He then described an oxide of 

 platinum obtained by the action of nitrous acid on fulminating 

 platinum. This oxide is of a grey colour, and, according to his 

 experiments, consists of 100 platinum + 11'9 of oxygen. He 

 considered this as a protoxide composed of 1 atom metal + I 

 atom oxygen ; while the black oxide of platinum he considered 

 as a compound of 1 atom metal + 1,1- oxygen. 



Feb. 24. — A paper, by Dr. WoUaston, was read, entitled, " On 

 the Method of cutting Kock Crystal for Micrometers." A wedge 

 of rock crystal, and one of crown glass, may be so combined for 

 ' the purposes of examining the phenomena of double refraction, 

 that the image of a luminous object seen through them shall 

 appear in its true place by ordinary refraction, accompanied by 

 a second image produced by extraordinary refraction. In conse- 

 quence, however, of the dispersion of col'"iurs, which arises from 

 employing different substances, such a combination is not 

 adapted for the micrometer invented by the Abbe Rochon ; but 

 it is not difficult to obtain such a section of rock crystal as may 

 be substituted for the glass wedge, so that the pencil of hght 

 shall be colourless without diminishing the separation of the 

 images. But since the distance to which the double refractioii 

 of rock crystal separates the two portions of a transmitted ray is 

 sometimes not sufficiently great, it becomes desirable to increase 

 it ; and thouHi the means of effectino- this have not been 

 published, the author proceeded to describe a method which he 

 found to succeed, and which he regarded as the same as that of 

 M. Rochon. He then described three modes of cutting wedges 

 of rock crystal, so that the axis of crystallization shall be diffe- 

 rently situated in each. In tlie first, or horizontal wedge, the 

 axis is at right angles with the surface. In the second, or 

 lateral wedge, the axis is in the first surface, and parallel to its 

 acute edge. In the third, or vertical wedge, the axis was also 

 in the first surface, but at rip'ht angles to the acute edg-e. 

 Through the first wedge an object is seen in the direction of the 

 axis, and does not appear double ; but in both the others, the 

 transmitted rays pass at a right angle to the axis, and they 

 produce two images. By placing two of these wedges together 

 with their acute edges in opposite directions, there are obviously 

 three modes in which they may be combined in pairs. In the 

 first two cases, the separation of the images will be the same, or 

 about 17' ; but the third produces a different effect ; for, by 

 reason of the transverse position of the axes of crystallization, 

 the separation of the two images seems exactly doubled. The 

 pencil ordinarily refracted by the first wedge is refracted extra- 

 ordinarily by the second, and vice verm; so that neither of the 

 divided pencils returns to its true place, and since one falls as 



