1896.] Mr Newall, On the Spectroscope, etc. 179 



Monday, 25 May, 1896. 



Professor J. J. Thomson, President, in the Chair. 



At a Meeting held at the Observatory the following com- 

 munications were made : 



(1) On the spectroscope used in connexion with the 2o-inch 

 refractor. By H. F. Newall, M.A., Trinity College. 



Mr Newall exhibited the spectroscope and shewed some of the 

 photographs of stellar spectra with terrestrial comparison spectra 

 taken with the instrument. A general description of the spectro- 

 scope has been published in the Monthly Notices of the Royal 

 Astronomical Society, 1896, vol. LVi. p. 98. 



(2) On a suggestion for a form of spectrohelioqraph. By 

 H. F. Neavall, M.A., Trinity College. 



The results obtained by Mr Hale in photographing the sun's 

 disc with the light of some definite wave-length emitted by it are 

 so full of interest and of promise of further development that it 

 seems pardonable to make an untried suggestion which may lead 

 to increased facility in taking such photographs. 



In a paper entitled " The Spectroheliograph " (Astronomy and 

 Astrophysics, 1893, p. 241) Mr Hale has given a resume of the 

 earlier attempts to photograph prominences and of his own work 

 in developing the spectroheliograph. (See also Astronomy and 

 Astrophysics, 1892, pp. 407, 603.) 



M. Deslandres has in a paper entitled " Appareils enregistreurs 

 de l'atmosphere solaire" (L'Astronomie, June 1894), considered 

 various forms of apparatus for researches similar to those carried 

 out by himself and by Mr Hale. (See also Bulletin astronomique de 

 I'Observatoire de Paris, Feb. 1894). 



The general principle of the method is as follows : 



An image of the sun is produced and allowed to fall on a slit ; 

 the illuminated part of the slit is a chord of the sun's disc. If the 

 proper relative motion of the slit and the sun's image is brought 

 about, the slit can be illuminated successively by parallel chords 

 of the sun's disc. 



The slit is connected with some spectroscopic (or rather spectro- 

 genic) apparatus which produces a monochromatic image of the 

 slit, and this image is isolated from the neighbouring images by 



