Page:Lectures on Ten British Physicists of the Nineteenth Century.djvu/144

 Carlyle, Carnegie, the choice of the students is guided by other than political reasons. In 1843 Herschel made a reproduction of an engraving of the Slough 40-foot reflector which was the first example of a photograph on glass. He was the first person to use the terms positive and negative for photographic reproductions. His discovery in 1845 of the "epipolic" dispersion of light produced by sulphate of quinine and some other substances led the way to Stokes' explanation of the phenomena of fluorescence.

In 1845 Herschel was called on to preside at the second Cambridge meeting of the British Association. Since his own student days, Cambridge had made great progress in mathematical science. The "d-ists"' had long since triumphed over the dot-ards. The Cambridge Philosophical Society had been founded for the reading and publication of scientific memoirs; the Cambridge Mathematical Journal had been founded; and the University Observatory had been made an up-to-date institution. His immediate predecessor in the chair was another "d-ist", George Peacock, now dean of Ely; and after the close of the meeting Herschel and Hamilton were guests at the deanery, on which occasion both essayed their poetic power. Two years before the Quaternion theory had been published; and Herschel referred to it in his presidential address. The closing passage of this address is characteristic of the man: "In these our annual meetings, to which every corner of Britain —almost every nation in Europe—sends forth as its representative some distinguished cultivator of some separate branch of knowledge; where I would ask, in so vast a variety of pursuits which seem to have hardly anything in common, are we to look for that acknowledged source of delight which draws us together, and inspires us with a sense of unity? That astronomers should congregate to talk of stars and planets— chemists of atoms—geologists of strata—is natural enough; but what is there of equal mutual interest, equally connected with and equally pervading all they are engaged upon, which causes their hearts to burn within them for mutual communication and unbosoming? Surely, were each of us to give utterance