288 PROCEEDINGS. 



old in years is full of vigour, and is now working hard to get an 

 Observatory furnished in a style which he thinks commensurate 

 with the work in hand. He has a large place, the old palace of 

 Napoleon at Meudon, to work upon ; and part of the ruined 

 building -for it was a stronghold of the Prussians in the Franco- 

 German war, stands upon a hill overlooking Paris. A truly 

 magnificent site for an Observatory, round it are the various 

 telescopes now in use ; but in the grand old building is to be 

 erected a refractor, with an object glass of 31^- inches, and a 

 focal length of 40 feet, so arranged that the ordinary objective 

 can at any moment be removed, and a photographic objective 

 of 23^ : inches diameter and 40 feet focus put in its place, and 

 used for photographing the sun, moon and stars. These things 

 are ordered, and M. Janssen waits impatiently' like other 

 astronomers until the mechanic makes what he wants." 



Mr. Russell, who was warmly welcomed on this his first 

 appearance at the Meetings of the Society since his return from 

 the Conference of Astronomers at Paris, said that one of the 

 difficulties about the photographs was to get the pictures perfectly 

 round. The focus of an ordinary lens has a curved surface, and 

 it is found that any expansion of the picture beyond one degree 

 produces a distortion in the image of the stars in the corners of 

 the photograph. The images indeed are oval and not circular. 



In reply to a question of Mr. Hirst as to the time of exposure 

 allowed for the sun photograph of M. Janssen, Mr. Russell stated 

 that M. Janssen was very reticent, and it was a mark of great 

 favour that he was allowed to see the photograph, a copy being 

 afterwards presented to each Member of the Conference. He 

 understood that the photograph was taken a number of years 

 ago, and nothing to equal it had been since obtained. The time 

 of exposure was probably not more than - 3 — oVoth part of a second. 



Mr. Adair on behalf of Prof. Threlfall exhibited an arrangement 

 which had been devised for the purpose of obtaining a nearly 

 perfect astatic combination of magnets for galvanometer purposes. 

 Since, for accuracy, it is necessary to magnetize the needles in 

 situ on their suspension, and the distance between them is not 

 very great compared with the dimension of the magnet used in 

 the magnetization, the operation must be performed simultane- 

 ously on both needles, otherwise partial demagnetization of one 

 of them will ensue. The apparatus consisted of two small electro 

 magnets, provided with some obvious adjustments, and arranged 

 so as to be as magnetically similar as possible. The conditions 

 for this similarity were mentioned and the use of the instrument 

 explained. Some fine quartz threads drawn by Mr. JBoy's method 

 were also exhibited. 





