.1 NEW METHOD OF RESEARCH. 25 



the sun's image be formed by mirrors instead of lenses. Thus the 

 telescope should be a horizontal reflector of the ccelostal type. The 

 aperture of the mirrors should be as great as possible, since the high 

 dispersion of the spectroheliograph, even in the most favorable cases, 

 will involve long exposures. 



Bnl the most important requisite is such a condition of the atmos- 

 phere as will give the finest possible definition of the solar image. 

 This would involve the establishment of the instruments at some par- 

 ticularly favorable site, where careful telescopic tests have shown the 

 definition of the solar image to be exceptionally tine. Such a site is 

 most likely he found in regions where the sky remains cloudless for 

 weeks at a time: a point of great importance, since at such a place the 

 various phases of changing phenomena could be followed up day after 

 day with the same instruments. A still more thorough study of con- 

 stantly varying solar phenomena could be secured through the coopera- 

 tion of a chain of suitably equipped observatories, so distributed in 

 longitude as to permit the sun to be kept continuously under ob- 

 servation. 



With such large spectroheliographs, employed with a well-defined 

 solar image of sufficient size, it should be possible to study not only the 

 vapors lying around and above sun-spots, but those which constitute 

 the spot itself. With sufficiently high dispersion, for example, it 

 ought to be possible to secure photographs with the slit set at different 

 points on some of the broadest of the 'widened' lines, giving sections 

 of the vapors of the dark umbra of the spot at different depths below 

 the surface. A comparative study of the forms of the umbra, as re- 

 corded in different widened lines, and of those bright forms which 

 must appear on photographs taken with Fraunhofer lines that are 

 weakened in intensity or transformed into bright lines in sun-spots, 

 should provide data for the solution of important questions relative to 

 the solar constitution. 



But the spectroheliograph, though promising to supply an exceed- 

 ingly powerful method of attack, is only one of many instruments 

 required in a comprehensive investigation of solar phenomena. Power- 

 ful spectroscopes, equaling or surpassing in resolving power the largest 

 instruments now employed in the physical laboratory, must be used 

 simultaneously with the spectroheliograph in a study of the various 

 vapors. In such a research the displacements of lines in various 

 regions, corresponding to the effect of pressure or to the motion of the 

 gases in the line of sight, would play a prominent part. This most 

 precise quantitative work must lie accompanied by a systematic record, 

 extending through many sun-spot periods, of the lines which are 

 widened in sun-spots. Furthermore, there should be accurate measure- 

 ments, with the bolometer or radiometer, of the heat radiated from 



