516 Royal Astronomical Society. 



but on boiling the liquid for a minute or two protocyanide of 

 gold was precipitated, and no gold remained in solution. By 

 analysis it appeared to consist of one equivalent of protocyanide 

 of gold with 2 of cyanide of potassium *. 



From these experiments we may conclude, — 1. That there 

 is a protocyanide of gold remarkable for being the most stable 

 of all the cyanides, except perhaps cyanide of palladium. 

 2. That tercyanide of gold is reduced to protocyanide by 

 boiling muriatic acid. 3. That the protocyanide combines 

 with ammonia and with cyanide of potassium. 



LXXVII. Proceedings of Learned Societies. 



ROYAL ASTRONOMICAL SOCIETY. 



[Continued from p. 308.] 



December 8, f|1HE following communications were read : — I. On 



1843. JL the Apparent Magnitudes of the Fixed Stars. 



By C. Piazzi Smyth, Esq. Communicated by Captain W. H. Smyth, 



R.N. 



The author complains of the want of information on the methods 

 of observing the apparent magnitudes of the stars, and of the little 

 attention which has been paid to the proposal of a prize for a suc- 

 cessful photometer (Memoirs, vol. i. p. 507), by the Astronomical 

 Society. 



He proposes to employ telescopic vision, and to measure the 

 degrees of brightness of every star by means of the obscuration 

 which is necessary to make it vanish. By this means, the necessity 

 of direct comparison between stars taken two and two is avoided, 

 and an absolute zero is established. 



For producing the obscuration, he proposes, in the first place, a 

 long wedge of blue coloured glass (with its prismatic qualities coun- 

 teracted by a similar transparent wedge), made to slide between the 

 object and eye-glasses, a little way out of focus. This wedge might 

 be fixed on the eye end of the telescope, mounted either in a micro- 

 meter frame, or made to move in the manner of a barometer scale. 



Another plan is, to have a coloured disc of glass in the tube, ca- 

 pable of sliding up and down in it, by which means the object will 

 be differently obscured, on account of the variation of the diameter 

 of the pencil of rays at different distances. 



The author then dwells on the method of observation, the means 

 of getting rid of the atmospheric effect, the establishment of a com- 

 mon unit of comparison, and the obviation of the practical difficulty 

 of obtaining a uniform rate of obscuration. 



* After the reading of this paper before the Chemical Society, a paper 

 on the same cyanide of gold, by Messrs. Glassford and Napier, was read, in 

 which the composition of the double cyanide of potassium and gold was 

 stated to be 1 eq. of protocyanide of gold, 1 of cyanide of potassium and 

 1 of water, and on carefully repeating my analysis I found their statement 

 to be correct. 



