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160 by paving the way for photography, and paid back its debt to astronomy by shortening the processes of its art, and faithfully recording the face of the heavens, as the most skilful draughtsman could not do. Truly, Herschel was a seer, whose imagination captured truth, though men less gifted mocked him as a dreamer. The equerry in Windsor Castle was justified in assuring Miss Burney that time would do justice to Herschel, as it had done to Newton.

Herschel's mistakes, in his subsequent inquiries, arose largely from his belief in Newton's theory that light-giving bodies, like the sun, emit infinitely small particles, which enter the eye and affect the retina so as to produce vision. Hence he spoke of the momenta of these particles. His contemporary, Dr. Thomas Young, maintained that light, like air, was produced by waves propagated at a vast rate of speed, and in immensely short lengths, through a universally diffused and infinitely rare medium, called ether A Frenchman, Fresnel, has got most of the credit of establishing this theory. But the third question asked and answered by Herschel in these papers about the sun was. Is light the same or different from heat? His experiments were carefully arranged and as carefully made, and the conclusion reached was that they are different. He also wrote two long papers on the coloured rings produced when two watch-glasses, or one and a plane glass, are pressed together so as to leave a thin plate of air between them. Amid