REPORT OF COMMITTEE ON OBSERVATORIES 63 



The sea of flame which surrounds the sun increases in brilliancy and 

 activit)^ As the spots continue to increase in number and area, 

 eruptive phenomena on a tremendous scale become more and more 

 frequent. At the time of maximum activity the violence of these 

 disturbances and the rapidity with which flames hundreds of thou- 

 sands of miles in height form and disappear surpass all comprehen- 

 sion. In view of these facts it is not surprising that the question 

 has been raised whether the total radiation of the sun does not 

 undergo variations corresponding in some measure with these vari- 

 ations in the violence of the phenomena visible on its surface. 



The solution of this problem, as has already been pointed out, 

 requires that observations be made under conditions not now avail- 

 able. With instruments of the modern type installed at a great 

 altitude and with similar instruments at a second station some thou- 

 sands of feet below, the principal conditions needed for the solution 

 of this question would be provided ; for it is necessary not only to 

 measure the intensity of the sun's heat from an elevation so great as 

 to eliminate most of the obstacles interposed by the denser and more 

 fluctuating portions of the earth's atmosphere, but also to arrive at 

 a more thorough understanding of the absorption which solar rays 

 undergo in passing through our atmosphere, and particularly to 

 determine the difference in the quality and degree of this absorption 

 at different levels. The study of the earth's atmosphere is of great 

 importance in connection with this research. Indeed, it is not 

 improbable that after the completion of a thorough investigation, 

 carried on simultaneously at the upper and lower stations and ex- 

 tending over a sufficient period of time, it would be possible to 

 accomplish all of the purposes of a solar observatory at a lower 

 station. 



When observed in the telescope the sun's disk is found to be much 

 more brilliant at the center than near the circumference The differ- 

 ence is so great that it was detected in the earliest observations of the 

 sun, made with the imperfect instruments of the time of Galileo. 

 This difference is due to an absorbing atmosphere which completely 

 surrounds the sun, and reduces the intensity of the light and heat 

 radiated outward through it. In a study of this absorption made 

 by Vogel in 1877 it was found that at the edge of the sun's disk only 

 about 13 per cent of the violet rays escape. The percentage of 

 transmitted light increases progressively for the blue, green, and 

 yellow rays, until it amounts to 30 per cent for the red. From these 

 results it has been concluded that if the absorbing atmosphere were 



