The Herschelian theory of the sun was adopted, and long retained by Sir John. He believed in a cool, solid interior globe sheltered by a succession of aërial envelopes, rent, locally and temporarily, by tornadoes of fire. The presence of inhabitants on the globe so circumstanced was credible to him, although he abstained from dwelling upon the advantages of their state. He carefully followed, however, the progress of solar science, and in 1864 explained his altered views in the Quarterly Journal of Science. He now regarded the sun as a wholly gaseous mass—a conclusion in which he was anticipated only by Father Secchi. He added that it must be largely composed of matter kept in an intermediate condition between liquid and vaporous by “high temperature and enormous pressure.” The spot-period, he suggested, might be that of a revolving meteoric ring with condensations.
He was vividly interested in the “willow-leaf” controversy, raised in 1862 by Nasmyth’s misinterpreted observations. The objects seen were simply Sir William Herschel’s “nodules”—the luminous elements of the sun, held by Sir John in 1867 “to be permanently solid matter, having that sort of fibrous or filamentous structure which fits them, when juxtaposed by drifting about, and jostling one against another, to collect in flocks as flue does in a room.” He concluded with the remarkable assertion that the sun has no real surface, “the density diminishing from that below the photosphere to nil in the higher regions, where the pressure is nil.”
Herschel’s “Cape Observations” stands alone in astronomical literature for the wide and permanent interest of its contents. They are exceedingly various. Chapters on Halley’s Comet, on Sun-spots, the Satellites of Saturn, Astrometry, the Constitution of the Southern Galaxy, are associated with discussions on the nature and distribution of nebulæ, with monographs of two, and incidental notes on many of these enigmatical objects. The volume is illustrated with over sixty beautiful steel engravings of nebulæ and clusters, of sun-spots, and of the comet.
The speculations it includes regarding the nature of nebulæ, deserve even now to be remembered. Sir John was, at the outset, an unwavering adherent of the theory developed by his father in 1811. They were composed, he held in 1825, of a “self-luminous; or phosphorescent material substance, in a highly dilated or gaseous state, but gradually subsiding, by the mutual gravitation of its molecules, into stars and sidereal systems.” His personal experience, however, ran counter to this view. In 1833 he had become convinced that a nebula is, in general, “nothing more than a cluster of discrete stars.”
The successful resolution into stars, with the great Parsonstown specula, of many nebulæ until then called irresolvable, carried him still further in the same direction. To him, as to other thinkers, the presence in space of a self-luminous cosmic fluid became more than doubtful. In his Presidential Address to the British Association in 1845, he dwelt with enthusiasm on the completion of the Rosse reflector—“an achievement of such magnitude, that I want words to express my admiration of it.” He regarded “as one of the grand fields open for discovery with such an instrument, those marvellous and mysterious bodies, or systems of bodies, the nebulæ.” Their frequent resolution, actual or indicated, with increased optical power, led him to attribute recalcitrance in this respect to the smallness and closeness of the stars of which they consist; he held them, in short, to be “optically, and not physically, nebulous.”
A new consideration was thus introduced into discussions on nebulæ. The whole burthen of accounting for their varieties in telescopic aspect need no longer be thrown upon differences of remoteness; diversities in the size and closeness of nebular molecules would answer the same purpose. So that pulverulent agglomerations, it was thought, might pass by insensible gradations into collections of truly sun-like bodies. All distinction between nebulæ and clusters was then abolished, the members of both classes consisting, like the sun’s photosphere, of shining granules, supported in an obscure medium, varying in real magnitude from floccules to great globes, while each vast compound body rotated en masse on an axis. Whatever the merits of this scheme, it at least harmonises with the now prevalent opinion that nebulæ and clusters belong to one unbroken cosmical series. “They are divided,” Mr. Cowper Ranyard wrote in 1893, “by no hard and fast line. The larger nebulæ may be described as groups of stars surrounded by bright nebulosity, and star-clusters as groups of stars surrounded by faint nebulosity.”
Herschel’s assimilation of nebulæ to clusters was not meant to apply to “those extraordinary objects resembling the wisps and curls of a cirrous cloud,” which confront the astronomer in Orion, Argo, and elsewhere. “The wildest imagination,” he said, “can conceive nothing more capricious than their forms. With their resolution,” he averred, “and that of elliptic nebulæ, the idea of a nebulous matter, in the nature of a shining fluid or condensible gas, would cease to derive any support from observation.” He, in fact, discarded it absolutely on the deceptive analysis into stars at Parsonstown and Harvard College of the Orion and Andromeda nebulæ. The discredited hypothesis was nevertheless triumphantly reinstated by Dr. Huggins’s spectroscopic observations in 1864.
One-third of the whole nebular contents of the heavens Herschel found to be collected into a broad, irregular patch, the central point of which in Virgo coincides almost precisely with the northern pole of the Milky Way. He compared it to a canopy surmounting the galactic zone. In the other hemisphere the arrangement, although less distinctly characterised, is on the same general plan. Plainly, then, nebular distribution has an opposite correspondence with stellar distribution, and the two partial systems are complementary one to another, Herschel, however, contented himself with the somewhat ambiguous statement that “the nebulous system is distinct from the sidereal, though involving and, to a certain extent, intermixed with it.”
His verdict as to the ground-plan of the sidereal edifice might be summed up in the phrase, “Not a stratum, but an annulus,” our own situation being in a relatively vacant interior space. Hence, the sun belongs, not to the Milky Way proper—as it should on the stratum theory—but to the system of which the Milky Way forms part. This conclusion was in itself a distinct advance towards the solution of an exorbitantly difficult problem. The grand question as to the remoteness of the star-clouds in that gleaming sky-girdle was definitely raised by it; and the question is not, in the nature of things, unanswerable. Herschel’s annulus was not a neat structure with a cylindrical section, but “a flat ring, or some other re-entering form of immense and irregular breadth and thickness.” It is cloven over one-third of its circumference; it is interrupted by huge chasms; it is bent, and shattered and broken, and probably set with tentacular appendages, giving rise, by their foreshortening, to very complex visual effects. All of which modifying circumstances Herschel implicitly recognised. He was the first to gather any direct intimations of the existence of that “solar cluster” which, guessed at by the elder Herschel, has of late assumed a sort of elusive reality. A zone of bright stars, including those of Orion, Canis Major, the Ship, the Cross, and the Centaur, struck him at once as a conspicuous feature in the scenery of the southern heavens. Its aspect led him to “suspect that our nearest neighbours in the sidereal system form part of a subordinate sheet, or stratum,” inclined at an angle of twenty degrees to the plane of the Milky Way. To Dr. Gould at Cordoba, in 1879, “few celestial phenomena” appeared “more palpable” than this projected star-belt; and, since it traces out a great circle on the sphere, the sun must be placed within it, and pretty accurately in its plane; yet the difficulty of associating it intimately with our particular star seems all but insurmountable.
Herschel’s minor and occasional writings were neither few nor unimportant. He contributed articles on “Isoperimetrical Problems” and “Mathematics” to Brewster’s Edinburgh Cyclopædia, and on “Meteorology,” “Physical Geography,” and “The Telescope,” to the eighth edition of the Encyclopædia Britannica. These last were printed separately as well. He edited in 1849 the Admiralty “Manual of Scientific Inquiry,” and criticised in the Edinburgh and Quarterly Reviews Mrs. Somerville’s “Mechanism of the Heavens,” Whewell’s “History of the Inductive Sciences,” Humboldt’s “Kosmos,” and Quetelet’s “Theory of Probabilities.” His addresses as President of the Royal Astronomical Society were models of their kind, and the same might be said of his memoirs of Baily and Bessel in the “Monthly Notices.” Most of them were collected in 1857, with his review articles, into a volume of “Essays;” and his attractive “Familiar Lectures on Scientific Subjects,” published in 1867, gave permanence to some popular discourses delivered in the school-house of Hawkhurst, as well as to articles from Good Words on Light and other subjects. No less than 152 papers by him are included in scientific repertories.