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POPULAR SCIENCE MONTHLY 



tions above the solar surface equaling or exceeding those attained by 

 hydrogen itself (Fig. 1, a). Their suitability for the purpose of promi- 

 nence photography is due to several causes, among which may be 

 mentioned their great brilliancy, their presence at the center of broad 

 dark bands which greatly diminish the brightness of the sky spectrum, 

 and the comparatively high sensitiveness of photographic plates for 

 light of this color. 



While fairly efficient from an optical point of view, the spectro- 

 heliograph of the preceding year had possessed many mechanical de- 

 fects. In a new instrument, devised for use with the twelve-inch 

 Kenwood telescope, these were overcome, and means of securing the 

 necessary conditions of the experiment were provided. The first trials 



Fig. 3. 



Eruptive Prominence of March 25, 1895. a, 10 h 34m. 

 b, 10 h 58 m . Height, 281,000 Miles. 



Height, 135,000 Miles. 



of the instrument, made in January, 1892, were entirely successful, 

 and the chromosphere and prominences surrounding the sun's disk 

 were easily and rapidly recorded ( Fig. 2 ) . The details of their struc- 

 ture were shown with the sharpness and precision characteristic of the 

 best eclipse photographs. And the opportunity for making such 

 records, previously limited to the brief duration, never exceeding seven 

 minutes, of a total solar eclipse, was at once indefinitely extended. 

 Thus it became possible to study photographically the slowly varying 

 forms of the quiescent, cloud-like prominences, and, to particular ad- 

 vantage, the rapid changes of such a violent eruption as is illustrated 

 in Fig. 3. 



But even before this primary purpose of the work had been accom- 

 plished, the possibility of making another and much more important 

 application of the instrument' had presented itself. A photographic 



