The general telescopic exploration of the Milky Way began and ended with the Herschels. Their great reflectors have been superseded by the photographic camera. This particular application of its versatile powers encountered special difficulties; but they were happily overcome by Professor Barnard in July, 1889. A six-inch portrait lens afforded the two chief requisites of a powerful light-grasp and an extensive field; and plates exposed with it for some three hours showed accordingly, for the first time, “in all their delicacy and beauty” (to quote Professor Barnard’s words), “the vast and wonderful cloud-forms, with their remarkable structure of lanes, holes, and black gaps, and sprays of stars, as no eye or telescope can ever hope to see them.” The work has since been continued by him and others, notably by Mr. Russell at Sydney, and by Professor Max Wolf at Heidelberg, so that the complete round of the “circling zone” will, before long, have its varied aspects permanently recorded. They frequently present strange and significant forms. Branching, leaf-like, spiral, elliptical structures abound; individual stars are disposed in circlets, streams, parallel rows, curves of sundry kinds. A “clustering power” of unknown nature is ubiquitously active; orderly development is in progress. A creative purpose can be felt, although it cannot be distinctly followed by the mind.
Herschel’s “sweeps” in southern skies were continued until January, 1838; but with frequent intermissions. He was ready for every interesting object that came in his way—comets among the rest. “Encke’s—yours,” he informed his aunt, October 24, 1835, “escaped me owing to trees and the Table Mountain, though I cut away a good gap in our principal oak avenue to get at it.” Four days later he caught sight of Halley’s comet at its second predicted return. But for the stellar aspect of this body his observations of it would have begun much earlier; for, in the absence of an exact ephemeris, it was impossible to pick it out from among the stars it long precisely counterfeited. “I am sure,” he said, “that I must often have swept with a night-glass over the very spot where it stood in the mornings before sunrise; and never was surprise greater than mine at seeing it riding high in the sky, broadly visible to the naked eye, when pointed out to me by a note from Mr. Maclear, who saw it with no less amazement on the 24th.”
“This comet,” he wrote to Miss Herschel, March 8, 1836, “has been a great interruption to my sweeps, and I hope and fear it may yet be visible another month.” It lingered on just two. He watched with astonishment the changes it underwent. “Within the well-defined head,” he wrote in his “Cape Observations,” “and somewhat eccentrically placed, was seen a vividly luminous nucleus, or rather, an object which I know no better way to describe than by calling it a miniature comet, having a nucleus, head, and tail of its own, perfectly distinct, and considerably exceeding in intensity of light the nebulous disc or envelope.”
This strangely organised body was a very Proteus for instability of form. It alternately lost and recovered its tail. It contracted into the likeness of a star, then dilated into a nebulous globe, which at last vanished as if through indefinite diffusion. The whole mass “seemed touched, seemed turned to finest air.” During one week at the end of January—it had passed perihelion November 16—Sir John estimated that the cometary Amœba had increased its bulk no less than forty times!
The paraboloidal form characteristic of this comet and many others, was to him “inconceivable,” apart from the play of repulsive, in addition to attractive forces; and he suggested that high electrical excitement due to vaporisation, if of the same kind with a permanent charge on the sun, would plausibly account for the enigmatical appearances he had witnessed. From their close study at Königsberg, Bessel had already concluded “the emission of the tail to be a purely electrical phenomenon.”
In March, 1836, Herschel attacked the subject of southern stellar photometry. Carrying further the “method of sequences,” he determined the relative brightness of nearly five hundred stars, which he disposed in order on a single descending scale, and linked on by careful comparisons to the northern stars, as they “lightened into view” on the homeward voyage. By the device of an “artificial standard star,” he was besides enabled to obtain numerical values for the lustre of each star examined, in terms of that of Alpha Centauri. Most important of all, he rectified the current system of magnitudes, and introduced a definite “light ratio,” which has since been extended, and more strictly defined, but not altered.
His “astrometer” gave Herschel the means of balancing the lustre of Alpha Centauri against full moonlight. The latter proved to be 27,500 times more powerful. And Wollaston having determined the ratio of moonlight to sunlight at 1/800000 (corrected by Zöllner to 1/600000), it became feasible to compare the brightness of any particular star, as we see it, with the brightness of the sun. Alpha Centauri, for example, sends us, according to Herschel, 1/22 thousand millionth of the light we receive from our domestic luminary. Moreover, when the distance of the star came to be measured (it amounts to twenty-five billions of miles), light received could at once be translated into light emitted. And the result has been to show that the components of this splendid binary are, taken together, four times more luminous than the sun. Through Sir John Herschel’s photometric researches, then, the real light-power of stars at known distances became an ascertainable quantity; and it is an element of great importance to astrophysical inquiries.
On January 10, 1837, he wrote from Feldhausen to his brother-in-law: “I am now at work on the spots in the sun, and the general subject of solar radiation.” The sun was just then at an exceptionally high maximum of disturbance. Spots of enormous size frequently obscured its disc. One was estimated by Herschel, March 29, 1837, to cover, independently of others, an area of 3,780 millions of square miles. So that it considerably exceeded in dimensions the great spot-group of February, 1892, the largest ever photographed at Greenwich. The study of a series of such phenomena led him to propound the “cyclone-theory” of their origin. It marked a decided advance in solar physics, if only because it rested upon the fact—until then unaccountably overlooked—that spot-production is intimately connected with the sun’s rotation. He regarded it as a kind of disturbance incidental to a system of fluid circulation analogous to the terrestrial trade- and anti-trade winds. “The spots,” he said, “in this view of the subject would come to be assimilated to those regions on the earth’s surface where, for the moment, hurricanes and tornadoes prevail; the upper stratum being temporarily carried downwards, displacing by its impetus the two strata of luminous matter beneath, the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the sun below.”
But the fundamental cause of our atmosphere’s flow and counter-flow is absent in the sun. The earth is heated from the outside, and therefore unequally; hence the air rushes along, turning westward as it goes, from the chilly poles to the torrid zone of vertical sunshine. No reason is, however, apparent why the solar equator should be hotter than the solar poles. That adduced by Herschel is certainly inadequate. He supposed that, by a retention of heat at the equator due to the accumulation there, consequent upon his rotation, of the sun’s absorbing atmosphere, a difference of temperature might be maintained sufficient to keep the solar trade-winds blowing. But the effect is too slight to be detected. And, in fact, the main drift of the photospheric layers is along parallels of latitude. Polar and equatorial currents are insignificant and uncertain.
Herschel and Pouillet contemporaneously, although at opposite sides of the globe, succeeded in 1837 in measuring the intensity of solar radiation. They were the first to apprehend the true bearings of the question, which in principle are simple enough. All that is required is to determine the heating effects, in a given time, of direct sunshine. Its despoilment by our air has, indeed, to be allowed for. Here the chief element of uncertainty comes in. Herschel put the loss at one-third the original thermal power of vertical rays; Pouillet pronounced it nearly one-half; Langley, using the most refined appliances, concludes it to be four-tenths. Striking an average between his own and the French results, Herschel calculated that, at the sun’s surface, a shell of ice forty feet thick would melt in one minute, the rate being reduced, at the distance of the earth, to an inch in two hours and twelve minutes. And it is now practically certain that this estimate was too small by about half its amount.