The study of the sun’s composition, and of the nature of the stupendous processes by which his ample outflow of light and heat is kept up and diffused through surrounding space, has in our time separated, 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 higher 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.13 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 marvellous 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 15th, 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 unhappy 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 quâ non is translucency. During the great “Indian eclipse” of August 18th, 1868, the variously colored lines were, by the aid of prismatic analysis, first described, which reveal the chemical constitution of the flamelike “prominences,” forming an ever-varying, but rarely absent, feature of the solar surroundings. Immediately afterwards, M. Janssen, at Guntoor, and Mr. Norman Lockyer, in England, independently realised a method of bringing them into view without the co-operation of the eclipsing moon. This was done by fanning out with a powerfully dispersive spectroscope the diffused radiance near the sun, until it became sufficiently attenuated to permit the delicate flame-lines to appear upon its rainbow-tinted background. This mischievous radiance – which it is the chief merit of a solar eclipse to abolish during some brief moments – is due to the action of the atmosphere, and chiefly of the watery vapors contained in it. Were our earth stripped of its “cloud of all-sustaining air,” and presented, like its satellite, bare to space, the sky would appear perfectly black up to the very rim of the sun’s disc – a state of things of all others (vital necessities apart) the most desirable to spectroscopists. The best approach to its attainment is made by mounting a few thousand feet above the earth’s surface. In the drier and purer air of the mountains, “glare” notably diminishes, and the tell-tale prominence-lines are thus more easily disengaged from the effacing lustre in which they hang, as it were suspended.
The Peak of Teneriffe, as we have seen, offers a marked exception to this rule, the impalpable dust diffused through the air giving, even at its summit, precisely the same kind of detailed reflection as aqueous vapors at lower levels. It is accordingly destitute of one of the chief qualifications for serving as a point of vantage to observers of the new type.
The changes in the spectra of chromosphere and prominences (for they are parts of a single appendage) present a subject of unsurpassed interest to the student of solar physics. There, if anywhere, will be found the key to the secret to the sun’s internal economy; in them, if at all, the real condition of matter in the unimaginable abysses of heat covered up by the relatively cool photosphere, whose radiations could, nevertheless, vivify 2,300,000,000 globes like ours, will reveal itself; revealing, at the same time, something more than we know of the nature of the so-called “elementary” substances, hitherto tortured, with little result, in terrestrial laboratories.
The chromosphere and prominences might be figuratively described as an ocean and clouds of tranquil incandescence, agitated and intermingled with waterspouts, tornadoes, and geysers of raging fire. Certain kinds of light are at all times emitted by them, showing that certain kinds of matter (as, for instance, hydrogen and “helium”14) form invariable constituents of their substance. Of these unfailing lines Professor Young counts eleven.15 But a vastly greater number appear only occasionally, and, it would seem, capriciously, under the stress of eruptive action from the interior. And precisely this it is which lends them such significance; for of what is going on there, they have doubtless much to tell, were their message only legible by us. It has not as yet proved so; but the characters in which it is written are being earnestly scrutinised and compared, with a view to their eventual decipherment. The prodigious advantages afforded by high altitudes for this kind of work were illustrated by the brilliant results of Professor Young’s observations in the Rocky Mountains during the summer of 1872. By the diligent labor of several years he had, at that time, constructed a list of one hundred and three distinct lines occasionally visible in the spectrum of the chromosphere. In seventy-two days, at Sherman (8,335 feet above the sea), it was extended to 273. Yet the weather was exceptionally cloudy, and the spot (a station on the Union Pacific Railway, in the Territory of Wyoming) not perhaps the best that might have been chosen for an “astronomical reconnaissance.”16
A totally different kind of solar research is that in aid of which the Mount Whitney expedition was organized in 1881. Professor S. P. Langley, director of the Alleghany observatory in Pennsylvania, has long been engaged in the detailed study of the radiations emitted by the sun; inventing, for the purpose of its prosecution, the “bolometer,”17 an instrument twenty times as sensitive to changes of temperature as the thermopile. But the solar spectrum as it is exhibited at the surface of the earth, is a very different thing from the solar spectrum as it would appear could it be formed of sunbeams, so to speak, fresh from space, unmodified by atmospheric action. For not only does our air deprive each ray of a considerable share of its energy (the total loss may be taken at 20 to 25 per cent. when the sky is clear and the sun in the zenith), but it deals unequally with them, robbing some more than others, and thus materially altering their relative importance. Now it was Professor Langley’s object to reconstruct the original state of things, and he saw that this could be done most effectually by means of simultaneous observations at the summit and base of a high mountain. For the effect upon each separate ray of transmission through a known proportion of the atmosphere being (with the aid of the bolometer) once ascertained, a very simple calculation would suffice to eliminate the remaining effects, and thus