SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. 1 59 



(2) Studies of various solar phenomena, particularly through 

 the use of powerful spectroscopes and spectroheliographs. 



(3) Photographic and spectroscopic investigations of the stars and 

 nebulae with a very powerful reflecting telescope, for the principal 

 purpose of throwing light on the problem of stellar evolution. 



The present opportunity for important advances in these three 

 departments of research is very unusual. Since the publication of 

 Year Book No. 2, Dr. Langley has offered reasons to believe that 

 an actual change in the amount of heat emitted by the sun occurred 

 in March, 1903. It is hardly necessary to say that a change in the 

 intensity of the sun's heat, if actually established, might have a 

 most important bearing upon many questions relating to the earth, 

 and, at the same time, be of capital interest in its relationship to 

 the problem of the solar constitution. Through the force of circum- 

 stances, Dr. Langley's observations have been made under the very 

 unfavorable conditions which obtain at Washington. If they could 

 be continued at a considerable altitude, at a point above the denser 

 and more fluctuating region of the earth's atmosphere, the question 

 as to what changes actually occur in the solar radiation could doubt- 

 less be answered in a thoroughly satisfactory manner. 



In the study of the phenomena of the sun's surface and atmos- 

 phere we again enter a remarkably fruitful field of research. 

 Within the past few years the instruments available for work in 

 this field have been greatly developed, and now only await applica- 

 tion on a large scale in order to secure a great number of new 

 results which have hitherto been entirely out of reach. But even 

 if the means were available for supplying the necessary instruments 

 to existing observatories, they could not be successfully employed 

 without atmospheric conditions much superior to those at present 

 available. In work of this nature, success depends upon the 

 perfect definition of the soiar image and the absence of those dis- 

 turbances from which the atmosphere at existing observatories is 

 almost never free. For this work, therefore, an elevated station in 

 a region of great atmospheric calm is absolutely essential. Further- 

 more, the site must be free from the disturbing factors which fre- 

 quently prevent good observations from being obtained on mountain 

 summits. 



In the third class of investigations required to complete the 

 program of a properly equipped solar observatory, similar possi- 

 bilities of advance exist. Within the past few years the remarkable 

 advantages of the reflecting telescope have been demonstrated. It 

 now only remains to construct a larg^ and powerful instrument 

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