3 88 THE POPULAR SCIENCE MONTHLY. 



1. The prominences are not mere optical illusions ; they are real 

 phenomena pertaining to the sun. . . . 



2. The prominences are collections of luminous matter of great 

 brilliance, and possessing remarkable photographic activity. This ac- 

 tivity is so great that many of them, which are visible in our photo- 

 graphs, could not be seen directly even with good instruments. 



3. Some protuberances float entirely free in the solar atmosphere 

 like clouds. If they are variable in form, their changes are so gradual 

 as to be insensible in the space of ten minutes. (Generally, but by 

 no means always, true.) 



4. Besides the isolated and conspicuous protuberances there is also 

 a layer of the same luminous substance which surrounds the whole 

 sun, and out of which the protuberances rise above the general level 

 of the solar surface. . . . 



5. The number of the protuberances is indefinitely great. In direct 

 observation through the telescope the sun appeared surrounded with 

 flames too numerous to count. ... 



6. The height of the protuberances is very great, especially when 

 we take account of the portion hidden by the moon. One of them 

 had a height of at least three minutes, which indicates a real altitude 

 of more than ten times the earth's diameter. . . . 



But their nature still remained a mystery ; and no one could well 

 be blamed for thinking it must always remain so to some degree. At 

 that time it could hardly be hoped that we should ever be able to 

 ascertain their chemical constitution, and measure the velocities of 

 their motions. And yet this has been done. Before the great Indian 

 eclipse of August 18, 1868, the spectroscope had been invented (it 

 was, indeed, already in its infancy in 1860), and applied to astronomi- 

 cal research with the most astonishing and important results. 



Every one is more or less familiar with the story of this eclipse. 

 Herschel, Tennant, Pogson, Rayet, and Janssen, all made substantially 

 the same report. They found the spectrum of the prominences ob- 

 served to consist of bright lines, and conspicuous among them were 

 the lines of hydrogen. There were some serious discrepancies, indeed, 

 among their observations, not only as to the number of the bright 

 lines seen, which is not to be wondered at, but as to their position. 

 Thus, Rayet (who saw more lines than any other) identified the red 

 line observed with B instead of C ; and all the observers mistook the 

 yellow line they saw for that of sodium. 



Still, their observations, taken together, completely demonstrated 

 the fact that the prominences are enormous masses of highly-heated 

 gaseous matter, and that hydrogen is a main constituent. 



Janssen went further. The lines he saw during the eclipse were 

 so brilliant that he felt sure he could see them again in the full sun- 

 light. He was prevented by clouds from trying the experiment the 

 same afternoon, after the close of the eclipse; but the next morning 



