Page:Popular Science Monthly Volume 26.djvu/409

Rh it might be said, into a science apart. Its pursuit is, at any rate, far too arduous to be conducted with less than a man's whole energies; while the questions which it has addressed itself to answer are the fundamental problems of the new physical astronomy. There is, however, but one opinion as to the expediency of carrying on solar investigations at greater altitudes than have hitherto been more than temporarily available.

The spectroscope and the camera are now the chief engines of solar research. Mere telescopic observation, though always an indispensable adjunct, may be considered to have sunk into a secondary position. But the spectroscope and the camera, still more than the telescope, lie at the mercy of atmospheric vapors and undulations. The late Professor Henry Draper, of New York, an adept in the art of celestial photography, stated in 1877 that two years, during which he had photographed the moon at his observatory on the Hudson on every moonlit night, yielded only three when the air was still enough to give good results, nor even then without some unsteadiness; and Bond, of Cambridge (U. S.), informed him that he had watched in vain, through no less than seventeen years, for a faultless condition of our troublesome environing medium. Tranquillity is the first requisite for a successful astronomical photograph. The hour generally chosen for employing the sun as his own limner is, for this reason, in the early morning, before the newly emerged beams have had time to set the air in commotion, and so blur the marvelous details of his surface-structure. By this means a better definition is secured, but at the expense of transparency. Both are, at the sea-level, hardly ever combined. A certain amount of haziness is the price usually paid for exceptional stillness, so that it not unfrequently happens that astronomers see best in a fog, as on the night of November 15, 1850, when the elder Bond discovered the "dusky ring" of Saturn, although at the time no star below the fourth magnitude could be made out with the naked eye. Now, on well-chosen mountain-stations, a union of these unhappily divorced conditions is at certain times to be met with, opportunities being thus afforded with tolerable certainty and no great rarity, which an astronomer on the plains might think himself fortunate in securing once or twice in a lifetime.

For spectroscopic observations at the edge of the sun, on the contrary, the sine qua non is translucency. During the great "Indian eclipse" of August 18, 1868, the variously-colored lines were, by the aid of prismatic analysis, first descried, which reveal the chemical constitution of the flame-like "prominences," forming an ever-varying, but rarely absent, feature of the solar surroundings. Immediately afterward, M. Janssen, at Guntoor, and Mr. Norman Lockyer, in England, independently realized a method of bringing them into view without the co-operation of the eclipsing moon.