The learned societies of Europe vied with each other in enrolling the name of Sir John Herschel; and he was nominated, in 1855, on the death of Gauss, one of the eight foreign members of the French Academy of Science. As we have seen, he received the Copley Medal from the Royal Society twice, their Royal Medal thrice, and from the Royal Astronomical Society, two Gold Medals and a testimonial. Compliments and homage, however, left him as they found him—quiet, intent, and unobtrusive.
Several portraits of him are in existence. One was executed in oils by Pickersgill for St. John’s College, Cambridge, at a comparatively early period of his life. It is here ([page 142]) reproduced from an admirable engraving. His later aspect is finely represented in a painting by his eldest daughter, Lady Gordon. The eyes in it are sunken, though brilliant; the shape of the head is concealed by a mane of grey hair. There is about it something of leonine grandeur, disjointed from leonine fierceness. It perpetuates, indeed, the countenance of a man replete with human tenderness.
CHAPTER X.
WRITINGS AND EXPERIMENTAL INVESTIGATIONS.
Could the whole of Sir John Herschel’s astronomical career be obliterated, and the whole of his contributions to pure mathematics be forgotten, he would still merit celebrity as a physicist. Experimental optics, above all, engaged his attention. “Light,” he himself said, “was his first love,” and he was never wholly forgetful of it. In 1830 he described himself as “forcibly drawn aside from his optical studies” by the claims of nebulæ and double stars. How strong he felt those claims to be, can best be understood by considering the firmness with which he averted his mind, out of regard to them, from the intricate and bewitching subject of his early devotion.
“I understand from Peacock,” Dr. Whewell wrote to him, June 19, 1818, “that you are untwisting light like whipcord, examining every ray that passes within half a mile, and putting the awful question, ‘Polarised, or not polarised?’ to thousands that were never before suspected of any intention but that of moving in a straight line.” These interrogatories brought out a remarkable diversity in the action upon light of quartz, and other similar substances, corresponding with the two different modes of crystallisation belonging to each of them. Here, in Lord Kelvin’s phrase, is “one of the most notable meeting-places between natural history and natural philosophy.”
The nascent science of spectrum analysis was materially promoted by Herschel. He noticed in 1819 the distinctive light-absorbing qualities of coloured media, studied the spectra of various flames, adverted to the definiteness and individuality of the bright lines composing them, and recommended their employment for purposes of chemical identification.
A year later, he developed and modified Brewster’s explanation of the colours of mother-of-pearl. They do not, like the iridescence of a fly’s wing, result from the interference of waves of light reflected from two closely adjacent surfaces, but from interference brought about by the finely striated texture of the shell’s surface, and a cast of the rainbow-tinted surface in black sealing-wax will display the same sheen of colour as the original. Herschel detected, however, a second more closely striated structure which cannot be impressed upon plastic matter.
Up to this time he accepted unreservedly the emission theory of light. But a candid study of Young’s and Fresnel’s writings produced a fundamental change in his opinions; and in an article on “Light,” written for the “Encyclopædia Metropolitana” in 1827, he expounded the undulatory theory with all the ardour of a neophyte. He brought thereby one of the grandest generalisations of science into universal currency, and enforced its acceptance by the cogency of his arguments, the logical order of his method, and the lucidity of his style. The treatise was translated into French by Quetelet; and no reader, Professor Pritchard remarked, “could escape the charm of the half-suppressed enthusiasm which carried him along.”
Whewell ranked him “among the very small number of those who, in the singularly splendid and striking researches of physical optics, had both added important experimental laws to those previously known, and weighed the relations of these discoveries to the refined and recondite theory towards which they seemed to point.” He contributed to the same Encyclopædia scarcely less brilliant essays on Heat, Sound, and Physical Astronomy.