THE 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 possessing 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 construction tends to render more minute and microscopic, so great is the distance 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 knowledge we therefore may not expect that the great flames and other evidences 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 permits this result to be accomplished is the spectroscope, used in con-