6 v POPULAR SCIENCE MONTHLY. 



undergoing changes and transformations well calculated to assist in 

 the explanation of problems which the laboratory can not solve. 



But to the philosopher and the student of nature as a whole, the sun 

 finds its highest interest in its relationship to the great problem of stellar 

 evolution. For the central body of our system is a star, resembling 

 in the closest way many of the stars of the sidereal universe, but pos- 

 sessing the unique distinction of comparative proximity to the earth. 

 Even in the most powerful telescopes, all the other stars appear as 

 minute points of light, which every improvement in telescope construc- 

 tion tends to render more minute and microscopic, so great is the dis- 

 tance of these stars from the observer on the earth. We have no reason 

 to believe that telescopes will ever be constructed so powerful as to 

 magnify a stellar image into an actual disk. With our present knowl- 

 edge we therefore may not expect that the great flames and other evi- 

 dences of eruptive phenomena, which we believe from inference to be 

 as characteristic of the stars as of the sun, will ever become visible. 

 We must therefore depend for a knowledge of such phenomena upon 

 the one star whose surface can be studied in detail. Armed with this 

 knowledge, we may trace out with the spectroscope successive. steps in 

 the development of a star, from its origin in a nebula, on through the 

 earlier stages typified by such stars as Sirius, to the condition attained 

 in the sun. In this object we seem to observe the culmination of stellar 

 life. For evidences of decay we must investigate the red stars, in 

 which the radiation of heat throughout immense periods of time has 

 resulted in cooling toward the point of final extinction. Thus we may 

 untangle the great problem of stellar evolution, and at the same time 

 build up a complete history of the sun, learning what it has been, what 

 it is and what it will become. 



Prior to the middle of the nineteenth century, at periods of total 

 eclipse, when the dark body of the moon cuts off completely the bright 

 light of the solar disk, red flames were observed at many points on the 

 moon's circumference. At first their nature was so little understood 

 that they were described by some observers as lunar mountains. But 

 in 1868, through the use of the spectroscope, their true gaseous nature 

 and their connection with the sun became known. It was found that 

 immense masses of hydrogen and helium gas rise from a sea of flame 

 (the chromosphere) which completely envelops the sun, and that these 

 'prominences' sometimes attain elevations of hundreds of thousands of 

 miles. 



The rarity and brief duration of total eclipses would have limited 

 greatly our knowledge of the prominences, had not a method been 

 devised by which they can be observed on any clear day in spite of the 

 glare of our atmosphere around the sun. The instrument which per- 

 mits this result to be accomplished is the spectroscope, used in con- 



