94 Proceedings, &c., for 1887. 



attainable. In taking the Paris photographs, not only was 

 clockwork used, but there was an auxiliary means of shifting the 

 telescope by hand. The mechanical means of getting the precise 

 motion required occupied the attention of the conference. He had 

 not received all the detailed reports of the proceedings, so he 

 scarcely knew what was finally decided upon in that direction. 

 The telescope required for the scheme had an aperture of 13in., in 

 diameter, and was about 13ft. in length, and the object glasses 

 had been made with optical properties such as would give good 

 photographic images on the plates. It was proposed to take in a 

 field of about four degrees for each plate, and the plates would be 

 arranged in a uniform order, and would overlap. It was expected 

 that about 4,000,000 stars would be charted, so that the 

 arrangement of the plates would be no light task. A great many 

 stars of small magnitude were photographed which could not be 

 seen by the eye, even with the best telescopes. It was very 

 possible that many more stars would be found on these jjlates than 

 could be seen. As to the movement of the telescope, a very 

 ingenious apparatus had been contrived by Mr. Grubb. If it 

 fulfilled expectations, one of the great difficulties in the way of 

 carrying out this great work would be surmounted. It had been 

 arranged that all the photographic plates should be made by one 

 maker, and it was agreed that it was very desirable that all 

 telescopes should be of one particular class and size, and, if 

 possible, by the same makers. It would be at least another year 

 before operations could be begun, and the work would occupy five 

 or six years. 



The President said with reference to the statement that stars 

 could be photographed, although invisible to the telescopically 

 assisted eye, the rays from them being actinic rather than 

 luminous, that it was a question whether some others might not 

 be lost if some are gained. He estimated that some 100,000- 

 photographs would have to be taken and compared. This would 

 give some idea of the enormous work involved. If it were 

 possible to obtain such a chart by any means 2,000 years old, it 

 would be of immense value to us now ; 2,000 years hence, 

 posterity will have such materials towards furnishing a history 

 of the heavens. 



Mr. White thought it scarcely possible to keep a star bisected 

 by mechanical means, on account of variations of temperature 

 constantly varying refraction. The work of cataloguing the stars 

 when photographed, will take many years to complete. As the 

 work must be done, the sooner it begins the better. 



Mr. Ellery mentioned that some years ago he had obtained 

 pliotogiaphs of different coloured stars in groups, which reversed 

 their relative sizes. Those photographs would be useful now. I£ 



