306 Sir Howard Grubb [May 25, 



discovery of spectroscopic double stars, stars so close that we cannot 

 hope to see them double by any possible optical means, and yet of 

 which we know sufficient by the aid of photography to calculate their 

 masses, distances, periods and rates of motion with considerable 

 accuracy. In this work Professor Pickering, at Cambridge, Mass., 

 and Dr. Yogel, at Potsdam, led the way, but Father Sidgreaves, 

 at Stonyhurst, has extended the work by his beautiful analysis of the 

 variations in the spectrum of (3 Lyrae. 



Lastly may be mentioned the work of Professor Hale at Chicago, 

 on the photography of solar prominences and faculae with the 

 spectro-heliograph. In this apparatus, instead of using the full 

 light of the sun, only light of one wave-length is allowed to act on 

 the plate. There are several methods of accomplishing this, but 

 the latest form is that of an ordinary spectroscope with a metal 

 plate to receive the spectrum, a narrow slit being arranged in 

 this plate to select the particular wave-length in which it is 

 desired to work. The photographic plate is behind a second slit 

 and in the image formed of the spectrum. The whole spectroscope is 

 given a motion such that the front slit passes over the image of the 

 sun formed by a photographic object glass, while the selecting slit 

 moves at the same rate in front of the photographic plate. A 

 complete picture of the prominences, chromosphere and faculse of 

 the sun is thus obtained, and by an ingenious adaptation of clock- 

 work Professor Hale has been able to make his apparatus automatic, 

 and to set it to take 36 plates of the sun at any desired interval of 

 time between each, without any superintendence whatever from the 

 observer in charge. Photograjmy has also been extended to the 

 study of solar spots, lunar and planetary detail, and many other 

 departments of astronomical research too numerous to mention, but 

 not having any special interest for us at present in their bearing 

 upon the instrumental arrangements. 



Every one of these branches of work has already been not only 

 suggested but put into actual practice with more or less success, but, 

 as usual in the inception of such work, there are many lessons to be 

 learned from the first few years' experience. The most evident fact, 

 and one easily learned from any one of the various branches 

 mentioned, is that the utmost perfection is necessary in the apparatus 

 which enables the telescope to follow the object to be photographed. 

 Before enumerating the various points necessary to be attended to to 

 ensure this accuracy, perhaps it would be well to explain why this 

 increased amount of accuracy is necessary when using the photo- 

 graphic method of observation. In the older methods it sufficed if 

 the star remained on the w r ires of the micrometer during the actual 

 observation, which rarely lasted many seconds, and even if the star 

 did move off the wire the observer could see that it did so, and would 

 move up his wire again to the star, repeat the observation, and would 

 not record it unless he was satisfied that all was right at the moment 

 of bi-section. 



