RUMFORD SPECTROHELIOGEAPH. 133 



hence its width must be such as exactly to inchide this line and to 

 exchide all light froui other parts of the spectrum. It is- evident 

 that the spectroheliogTaph may he considered simply as a form of 

 monochromatic exposing shutter, ditfering from the ordinary focal 

 plane shutter only tlirough the use of a narrower exposing slit and 

 the inclusion of an optical train which limits the light to a single 

 line in the spectrum. 



Although this idea suggested itself to me quite independently in 

 1889, I subsequently learned that the principle was by no means new. 

 Indeed, Janssen had suggested it as early as LSGO, Avhih' Braun, of 

 Kalocsa, and Lohse, of Potsdam, had designed instruments involv- 

 ing the same principle in 1872 and 1880, respectively. Indeed, 

 Lohse had constructed and experimented with the instrument he de- 

 signed, but his work was not successful. This may have been due 

 in part to the fact that the hydrogen line which he employed is not 

 nearly so well adapted for prominence photography as are the H 

 and K lines of calcium. This was one of the difficulties experienced 

 in my first (unsuccessful) experiments, which, through the kindness 

 of Professor Pickering, Avere made at the Harvard College Observa- 

 tory in the winter of 1889-1890. 



In April, 1891, after the Kenwood Observatory had been equipped 

 with a 12-inch equatorial refractor and a powerful solar spectroscoj^e, 

 a photographic study of the ultra-violet spectrum of the chromosphere 

 and prominences was undertaken in the hope of finding lines better 

 adapted than those of hydrogen for the photography of the promi- 

 nences. The brilliant II and K lines of calciiuu, previously observed 

 visually in full sunlight by Professor Young and photographically at 

 total eclipses, were found in all cases to be the most conspicuous lines 

 in the spectrum of the chromosphere and prominences. The remark- 

 able brightness of these lines, and more particularly their position at 

 the center of the dark, broad shades, due to the denser calcium vapor 

 in the lower portion of the solar atmosphere, render them peculiarly 

 well adapted for the purposes of prominence photography. Indeed, 

 it was possible with their aid to obtain good photographs of single 

 prominences merely by opening the slit of the spectroscope to such an 

 extent as to include a considerable part of the prominence and giving 

 a very short exposure to the image formed directly upon a photo- 

 graphic plate. But this method was too limited to be of general 

 application. In order to record photographically the entire surface 

 of the sun, with the chromosphere and prominences, it was necessary 

 to employ the principle of the spectroheliograph, involving the use 

 of narrow slits, moved with reference to the solar image and i)hoto- 

 graphic plate. The first successful spectroheliograph Avas brought 

 into use at the Kenwood ObserA^atory in January, 1892. After this 

 t-ime it Avas employed regularly on cA'ery clear day until May, 1895, 



