144 RUMFORD SPECTROHELIOGRAPH. 



simultaneous observations, both visual and j)hotogTapliic, with other 

 instruments are essential. For example, while a photograph is being 

 taken Avith the spectroheliograph exposures on the spectrum of the 

 region under investigation, for the purpose of showing the widened 

 lines in sun spots as well as the motions in the line of sight of the 

 calcium vapor in the flocculi, should be provided for. These must 

 be made with an instrument of sufficient dispersion to permit the 

 photographs to be measured with high precision. At the same time 

 large scale ]3hotographs of the photosi^here and spots, made by direct 

 photographic methods, are needed for comparison with the spectro- 

 heliograph results. Many bolometric studies are also re(|uired, as 

 well as numerous other investigations wliich will suggest themselves 

 to the reader. The essential point is that a simultaneous attack 

 should be made on solar phenomena with a series of powerful instru- 

 ments, each designed to ansAver definite questions, and thus to furnish 

 some of the material that will be required for solving solar problems. 



But such an attack, comprehensive as it might be made, would be 

 greatly hampered if the atmosphei'ic conditions w^ere not favorable. 

 The difference between the effects of good and bad seeing may be 

 seen by comparing the blurred photograph reproduced in fig. 1, 

 j)late VII, Avhich Avas the best that could be obtained at the time, Avith 

 the (larger scale) photograph reproduced in plate vi, Avhich Avas 

 made Avith precisel}^ the same instruments and adjustments at a time 

 when the conditions Avere unusually good. If such conditions as 

 these latter could be had day after day for long periods of time, with 

 occasional periods of eA^en finer definition, many questions noAv out 

 of reach could be solved. 



A report on the instrumental and atmospheric conditions needed 

 in future Avork on the sun may be found in the forthcoming Year- 

 book (No. 2) of the Carnegie Institution. 



October, 1903. 



