144 



RUMFORD SPECTROHELIOaKArH. 



vsimultaiieous observations, both visual and photographic, Avith other 

 instninients are esseiitiaL For example, while a photograph is being 

 taken with the spectroheliograph exposures on the spectrum of the 

 region under investigation, for the purpose of showing the wddened 

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

 calcium A^apor in the flocculi, should be provided for. These must 

 be made with an instrument of suflicient dispersion to permit the 

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

 large scale photographs of the photosphere and spots, made by direct 

 photographic methods, are needed for comparison wdth the spectro- 

 heliograph results. Many bolometric studies are also required, as 

 well as numerous other investigations wlfich will suggest themseh^es 

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

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

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

 some of the material that Avill l)e required for solving solar problems. 



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

 greatly hampered if the atmospheric conditions were not favorable. 

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

 seen by comparing the blurred j)hotograph reproduced in fig. 1, 

 plate VII, Avhich Avas the best that could be obtained at the time, with 

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

 made with precisely the same instruments and adjustments at a time 

 when the conditions were 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 now out 

 of reach could be soh^ed. 



A report on the instrumental and atmospheric conditions needed 

 in future w^ork on the sun may be found in the forthcoming Year- 

 book (No. 2) of the Carnegie Institution. 



October, 1903. 



