“Do not observe too much in cold weather,” Miss Herschel advised her nephew, in anticipation of the winter of 1831–2; “write rather books to make folks stare at your profound knowledge.”
He followed the positive part of her counsel. Indeed, his “Preliminary Discourse on the Study of Natural Philosophy” had made its appearance in the previous year, as the introductory volume to Lardner’s “Cabinet Cyclopædia.” It was greeted with a chorus of approbation. Gauss reviewed it in the Gelehrte Anzeigen, Whewell in the Quarterly Review. Translated into French, German, and Italian, it delighted “all sorts and conditions” of readers with the justice and breadth of the views set forth in it agreeably, easily, and without pretension to superiority. The book included a survey of the actual state of scientific knowledge, and a philosophy of its augmentation. Students derived from it, Gauss remarked, both information as to how accepted results had been obtained, and guidance for their personal investigations. Herschel was exceptionally qualified, Whewell wrote, “to expound the rules and doctrines of that method of research to which modern science has owed its long-continued, steady advance, and present flourishing condition.” He had the knowledge, without the narrowness, of a specialist in almost every department of experimental physics. “With singular alacrity,” he came to the front wherever there seemed a chance of pushing back the barriers of ignorance. A disciple of Bacon, he had the advantage over his master of being habitually conversant with the practical working of inductive methods. The treatise was styled by Whewell “an admirable comment on the ‘Novum Organum.’” One, however, possesses the indefinable quality of greatness; it stands out from the centuries a solid structure, clothed with visionary magnificence; the other is elegant, attractive, wise, acute, even profound, but not in any degree, or from any point of view, great.
It was followed, in 1833, by “A Treatise on Astronomy,” published in the same series. An “Edinburgh Reviewer” (doubtless Brougham once more) perused it with regret. “The proper position of Sir John Herschel” he considered to be “at the head of those who are nobly, though it may be silently and without notice, endeavouring to extend the present limits of human knowledge,” rather than among “the ranks of those whose office it is to herald the triumphs of science, and point out its treasures and results to the admiration of the vulgar.” This ostensibly flattering estimate was made the basis for an imputation of vanity. The inducements, according to the critic, were strong “to descend from the airy summits of abstract science to the level at which the great body of the reading public can appreciate and applaud. Philosophers, like other writers, naturally wish to be read, and to have reputation; and reputation, as was remarked by d’Alembert, depends more upon the number than the merit of those who praise.” Sir John Herschel would have been better employed in pursuing the track of original discoveries, leaving new truths to “find their way to the drawing-room as best they might.” The whole tenour of his life refuted these insinuations.
The “Treatise on Astronomy” was enlarged in 1849 into the deservedly famous “Outlines of Astronomy.” Twelve editions of this book were published, the last in 1873; it was translated into Chinese and Arabic, as well as into most European languages, including Russian; it made a profound and lasting impression upon the public mind. No science has perhaps ever received so masterly a general interpretation. Methodical in plan, inspiriting in execution, it demands readers willing to share some part of the pains, for the sake of partaking in the high pleasures of the writer. For it is popular in the sense of eschewing mathematical formulæ, not in the sense of evading difficulties.
The work fittest to be set by its side is the “Exposition du Système du Monde.” But Laplace restricted his view to the sun’s domain, while Herschel excluded from his no part of the sidereal universe. Laplace was, besides, a geometer in the first, an astronomer only in the second place. The movements of the heavenly bodies interested him because they afforded opportunities for analytical triumphs. Their intricacy notwithstanding, he was elated to find that they could not baffle his ingenuity in constructing formulæ to correspond. Their balance, their harmony, their obedience to a single and simple law, gratified the orderly instincts of his powerful yet frigid mind. Where he could not explain, however, he did not admire. Mystery had no attraction for him. Knowledge, to be knowledge in his eyes, should have definite, clear-cut outlines. His scheme of the universe was like the map of the world laid down by Hecatæus, neatly finished off with a circumfluent ocean-stream; it included no intimations of a beyond. Herschel’s, on the contrary, might be compared to the map of Herodotus, in which some details were filled in, while the external boundary had been abolished. The most essential part of the progress made in the interval consisted in leaving verge and scope for the unknown. Next to nothing remained to be learned of the heavens, as they presented themselves to the author of the “Mécanique Céleste”; while Herschel saw everywhere only beginnings, possibilities of discovery, and dim prospects of “ultimate attainments,” as to the realisation of which “it would be unwise to be sanguine, and unphilosophical to despair” (Playfair). At the head of very many of his chapters he might, without presumption, have written: “Quorum pars magna fui.” They gave largely the results of his personal investigations, and were vivified by immediate acquaintanceship with the objects described. Hence the unsought picturesqueness of his descriptive epithets, and the sublimity of trains of thought communicated to him direct from the unveiled heavens.
Herschel invented in 1825, jointly with Babbage, the “astatic,” or neutralised magnetic needle—a little instrument which was no sooner available than it was found to be indispensable. “Nihil tetigit quod non ornavit.” And many and various were the things touched by his versatile genius. He had a narrow escape of becoming for life a chemist. At the very outset of his career he applied for the vacant chair of that science at Cambridge; but was left, as he himself humorously expressed it, “in a glorious minority of one.” The chemical inquiries, nevertheless, which he carried on at Slough brought to his notice one set of relations of no trifling importance. This was the solvent effect upon salts of silver of the hyposulphites of soda, potash, etc. The discovery was turned to account by himself in 1840 for the “fixing” of photographic images. It secured the future of the embryo art. By the agency of hyposulphite of soda in washing away the unaffected chloride of silver, while leaving untouched the parts of the deposit decomposed and darkened by exposure, permanent light-pictures, capable of indefinite multiplication, were at length secured.
On March 14th, 1839, unaware that he had been anticipated by Fox Talbot, Herschel presented to the Royal Society twenty-three prints made by the sensitised paper process. A memoir communicated in 1840 was full of suggestive novelties. In it he described experiments on “the chemical analysis of the solar spectrum,” pointing out that the character and amount of the action exercised by the various rays depend mainly upon the nature of the substance acted upon. He made a start, too, with spectral photography, and his detection of the “lavender-grey” effect to the eye of the ultra-violet section might be said to have added a new note to the prismatic gamut. In the opposite, or infra-red end, by simply letting the solar spectrum fall upon a strip of paper moistened with alcohol, he detected, through the different rates of drying where they fell, some of the “cold bands,” by which the invisible heat-rays are furrowed. The photo-spectroscopic apparatus devised for the purpose of these researches formed part of the Loan Collection of Scientific Instruments exhibited at South Kensington in 1876.
Still more essential was the improvement of substituting for paper, glass plates spread with a sensitive film. A photograph of the old forty-foot telescope, taken by this method in 1839, and preserved in the South Kensington Museum, is of unrivalled antiquarian value as regards the history of photography. The terms “positive” and “negative” received in this remarkable paper their now familiar photographic meaning. Its merits were acknowledged in 1840 by the award of a Royal Medal.
Sir John Herschel would, doubtless, at that time have set aside as a chimera the notion that the art he was engaged in promoting was destined, in large measure, to supersede visual methods in astronomy; that the great telescopes of the future would find their most useful employment in concentrating the rays of celestial objects upon sensitive plates. He soon perceived, however, the importance of photography as an adjunct to direct observation, and recommended, in 1847, the automatic self-registration of sun-spots. This hint—emphasised in 1848—was acted upon in 1858, when the regular collection of documentary evidence as to the sun’s condition was begun at Kew with De la Rue’s “photoheliograph.”
In 1845 he published the first effective investigation of “fluorescence,” called by him “epipolic,” or superficial, “dispersion.” This curious phenomenon consists in the illumination to the eye of certain substances, such as sulphate of quinine and canary glass, under the play of invisible light. Sir George Stokes showed in 1852 that the impinging rays have their undulations actually lengthened by the action of such kinds of matter, so as to become degraded in the spectrum, and thus brought within the range of vision.