ASTRONOMY — HALK. 67 



.STELLAR SPECTROSCOPY (Messrs. Halk and Adams). 



One of the principal objects of the Solar Observatory is to secure 

 photographs of the spectra of certain bright stars with a long focus 

 grating spectrograph on a scale comparable with that of Rowland's 

 photographs of the solar spectrum. It is hardly necessary to say 

 that such photographs, if sharply defined, would be of the greatest 

 service in an investigation of the physical condition of various types 

 of stars and their relationship with the sun and with one another. 

 On account of the great length and weight of suitable grating 

 spectrographs and their liability to flexure, it is impossible to 

 attach them to equatorial telescopes. Consequently, in the absence 

 of powerful horizontal telescopes, such investigations as that here 

 described have not previously been undertaken. 



Evidently the principal difficulty to overcome is the extreme 

 faintness of the star's light as compared with that of the sun. The 

 importance of using a telescope of large aperture is therefore obvious, 

 but in any case the exposure of the photographic plate must be very 

 long. Hence the spectrograph must be rigidly mounted on a heavy 

 pier in a room where the temperature can be kept nearly constant 

 throughout the exposure. If no change occurs in the relative posi- 

 tions of slit, lenses, grating, and plate, and if the grating itself is 

 maintained at a constant temperature within very narrow limits, the 

 spectral lines should occupy precisely the same positions on the plate 

 whenever the star is on the slit. In extreme cases the orbital and 

 diurnal motions of the earth must be taken into account, but these 

 need cause no trouble. Thus the exposure may be prolonged until 

 the feebly luminous image has registered itself on the plate. 



The spectrograph used in the work with the Snow telescope has 

 collimating and camera lenses of 5 inches aperture and 13 feet focal 

 length, mounted rigidly, with the slit, grating-mount, and plate- 

 holder, on a single massive stone pier. The large plane grating, for 

 the use of which we are indebted to the kindness of Professor Ames, 

 of Johns Hopkins University, was ruled on Rowland's engine many 

 years ago. Although fairly bright in the first order, the spectra are 

 much less brilliant than those of the best gratings of recent years. 

 However, as no other large grating could be had, we were very glad 

 to be favored with the use of this one. The grating is mounted in 

 the front of a cubical brass box and its rear surface is bathed by, 

 water, which is constantly stirred by paddles revolved at slow speed 

 by a small electric motor, supported on the wall of the room. Special 

 precautions are taken to prevent vibration of the grating from the 



