REPORT OF COMMITTEE ON OBSERVATORIES 59 



to be measured, from which valuable conclusions could be drawn 

 as to their physical nature, and it would furnish the essential means 

 of studying stellar spectra on a larger scale than that afforded by 

 existing instruments. 



Up to the present time such powerful grating spectroscopes as 

 those of Rowland have not been employed for the study of stellar 

 spectra. As already stated, the spectroscopes are too large to be 

 adapted to equatorial telescopes, and the feeble light of the stars 

 would demand exposures far longer than can be given under present 

 circumstances ; but with a great reflecting telescope, so mounted 

 as to produce the image of a star in a constant temperature labora- 

 tory, there should be no serious difficulty in photographing the 

 spectra of the brightest stars with the most powerful grating spec- 

 troscopes. The exposures might have to be prolonged for several 

 nights in succession, but it would only be necessary during this 

 time to maintain the spectroscope rigidly mounted on fixed piers at 

 a constant temperature. With such photographs of stellar spectra 

 a large number of problems, of great importance in connection with 

 questions of solar physics, could be solved. For example, it would 

 be possible to determine beyond question whether, as is now believed 

 by some investigators, the red stars have on their surface a great 

 number of spots like those of the sun. If the presence of numerous 

 sun spots on these stars could be proved it would follow that as the 

 sun grows colder, and advances toward the condition of the red stars, 

 the spots on its surface will multiply in number. Such a conclusion 

 would have a very important bearing on the problem of the solar 

 constitution. Many other similar questions could be answered, 

 such as those which relate to the relative pressure in solar and stellar 

 atmospheres, the changes in the solar spectrum which will result 

 from decreasing temperature, etc. 



General Nature of the Principal Problems oe Solar Research. 



It should be evident from what has been said that improved in- 

 struments and methods of research, employed under atmospheric 

 conditions more favorable than those experienced at existing observ- 

 atories, should render important advances possible. Let us now con- 

 sider briefly some of the principal problems of a solar observatory. 



The Constitution of the Sun. 



The problem of the solar constitution, though repeatedly attacked 

 on both observational and theoretical grounds, still remains unsolved. 



