His telescope, in fact, then held the championship. It was constructed in 1820 by himself, under his father’s directions, on the “front view” plan, the speculum being eighteen inches in diameter, and of twenty feet focal length. With it he executed, in 1824, a fine drawing of the Orion Nebula, with which “inexplicable phenomenon” he was profoundly impressed. It suggested to him no idea of a starry composition, and he likened its aspect to that presented by the “breaking up of a mackerel sky, when the clouds of which it consists begin to assume a cirrous appearance.”
In July, 1828, he succeeded in discerning the two Uranian satellites, Oberon and Titania, authentically discovered by his father. They had not been seen, except incidentally at Slough, for thirty years. His pursuit of them, continued at intervals until 1832, had the result of confirming, while slightly correcting, Sir William Herschel’s elements of their motions. On September 23, 1832, he perceived Biela’s comet as a round, hazy object without a tail. It closely simulated a pretty large nebula. A small knot of very faint stars lay directly in its path, and, having before long overtaken them, it “presented, when on the cluster, the appearance of a nebula partly resolved into stars, the stars of the cluster being visible through the comet.” They shone undimmed, he estimated, from behind a veil of cometary matter 50,000 miles thick. Yet, only a month later, the remote prospect of a collision with this tenuous body threw Europe into a panic.
After Sir William Herschel’s death, his son formed the project of collecting into a memorial volume all his published papers; but he decided before long that he could add more to his fame by pursuing and verifying his observations than by reprinting them. The keynote of his life’s activity was struck in these words. His review of the 2,500 Herschelian nebulæ, more than half of which were invisible with any instrument except his own, was begun in the summer of 1825, and terminated in 1833. The assiduity with which it was prosecuted appeared by its completion in little more than half the time judged necessary for the purpose by the original discoverer. Yet he was not exempt from discouragement. “Two stars last night,” he wrote, July 23, 1830, “and sat up till two waiting for them. Ditto the night before. Sick of star-gazing—mean to break the telescopes and melt the mirrors.” Very few glimpses of this seamy side to the occupation are afforded us by either of the Slough observers. Modern astronomers, by comparison, would seem, like the Scotchman’s barometer, to have “lost all control over the weather.”
The efficacious promptitude with which John Herschel swept the skies appears truly wonderful when we remember that he was without a skilled assistant. No ready pen was at hand to record what he saw, and how he saw it; he was, by necessity, his own amanuensis; and writing by lamplight unfits the eye for receiving delicate impressions. Yet a multitude of the objects for which quest was being made were of the last degree of faintness. The results were none the less admirable. Embodied in a catalogue of 2,307 nebulæ, of which 525 were new, they were presented to the Royal Society July 1, 1833, and printed in the Philosophical Transactions (vol. cxxiii.). Annotations of great interest, and over one hundred beautiful drawings, enhanced the value of the memoir.
Herschel was struck, in the course of his review, by the nebulous relations of double stars. A close, faint pair at the exact centre of a small round nebula in Leo; stellar foci in nebular ellipses; and a strange little group consisting of a trio of equidistant stars relieved against a nebulous shield, were specimen-instances illustrating “a point of curious and high physical interest.”
He also drew attention to “the frequent and close proximity to planetary nebulæ of minute stars which suggest the idea of accompanying satellites. Such they may possibly be.” If so, their revolutions might eventually be ascertained; and he urged the desirability of exact and persistent determinations of the positions of these satellite-stars. “I regret,” he concluded, “not having sufficiently attended to this in my observations, the few measures given being hurried, imperfect, and discordant.” Up to the present, these supposed systems have remained sensibly fixed; but they have been a good deal neglected. Mr. Burnham’s observations, however, with the Lick refractor in 1890–1, may supply a basis for the future detection of their movements in periods probably to be reckoned by millenniums.
The orbital circulation of compound nebulæ must be at least equally slow. They are most diverse in form and arrangement. “All the varieties of double stars as to distance, position, and relative brightness,” Herschel wrote, “have their counterparts in nebulæ; besides which, the varieties of form and gradation of light in the latter afford room for combinations peculiar to this class of objects.” Such, for instance, as the disparate union of an immensely long nebulous ray in Canes Venatici with a dim round companion, a small intermediate star occupying possibly the centre of gravity of the system.
Herschel’s drawings of double nebulæ have gained significance through their discussion, in 1892, by Dr. T. J. J. See of Chicago. They are now perceived to form a series aptly illustrative of the process, theoretically investigated by Poincaré and Darwin, by which a cooling and contracting body, under the stress of its consequently accelerated rotation, divides into two. If it be homogeneous in composition, its “fission” gives rise to two equal masses, presumed to condense eventually into a pair of equal stars. Disparity, on the other hand, between the products of fission indicates original heterogeneity; so that a large nebula must be of denser consistence than a smaller one physically connected with it. The chemical dissimilarity of the stars developed from them might explain the colour-contrasts often presented by unequal stellar couples. This view as to the origin of double nebulæ, and through them of double stars, although doubtless representing only a fragment of the truth, gives wonderful coherence to Herschel’s faithful delineations of what his telescope showed him.
No one before him had completely seen the “Dumb-bell” nebula in Vulpecula. Sir William Herschel had perceived the “double-headed shot” part of this “most amazing object,” but had missed the hazy sheath which his successor noticed as filling in the elliptic outline. He perceived similarly (unaware of Schröter’s observation) that the interior of the Ring-nebula in Lyra is not entirely dark; and compared the effect to that of fine gauze stretched over a hoop. An exceedingly long, nebular ellipse in Andromeda, with a narrow interior vacuity, left him “hardly a doubt of its being a thin, flat ring of enormous dimensions, seen very obliquely.” A photograph taken by Dr. Roberts, in 1891, corresponds strikingly with Herschel’s drawing. Some specimens of “rifted nebulæ,” were also included in the collection of 1833. They are double or even triple parallel rays, fragments, apparently, of single primitive formations. Herschel might well assert that “some of the most remarkable peculiarities of nebulæ had escaped every former observer.”
Both by the Royal, and by the Royal Astronomical Societies, medals were, in 1836, adjudged to this fine work. Its progress was accompanied by the discovery of 3,347 double stars, as well as by the re-measurement of a large number of pairs already known. The whole were drawn up into eight catalogues, presented at intervals to the Astronomical Society, and printed in their Memoirs. A good many of them would, nevertheless, be rejected by modern astronomers as “not worth powder and shot,” the stars composing them being too far apart to give more than an infinitesimal chance of mutual connection. From May 1828 onwards, these measures were made with “South’s ci-devant great equatorial,” purchased by Herschel. The object-glass, by Tulley, was five inches in diameter. With a twelve-inch refractor, its successor in South’s observatory on Campden Hill, Herschel detected, on its trial-night, February 13, 1830, the sixth star in the “trapezium” of Orion. This minute object was then about one-third as bright as the fifth star in the same group, discovered by Robert Hooke in 1664, but forgotten, and re-discovered by Struve in 1826. A slow gain of light in Herschel’s star is not improbable.