first curiosity once satisfied, he gave himself up to new curiosities. He resembled a traveler who glances over a virgin country, gives it a name, and hastens on to more distant horizons. All phenomena excited his imagination without holding him long. An experimenter by inspiration, a discoverer by instinct, he had too much passion, too little leisure, too fertile a spirit, but not enough of that obstinate perseverance that finishes what is begun.... Of theoretical ideas which include a whole science in a few general hypotheses, and leave an ineffaceable trace, he produced none, but sometimes repelled them, even when his own experiments led others to them." Biot, his former collaborator, took up Arago's experiments and worked them out in detail to the discovery of those more particular properties of polarization and the two rotatory powers which have been found of such useful application in the arts.
But neither Arago nor Biot was destined to work out the undulatory theory of light in all its significance and to the full explanation of the phenomena. That part fell to the young engineer Fresnel, who, rusticating in a village near Caen, in expiation of some political errors, passed his time in studying optics. He wrote to Arago, and received in return advice by which he profited so well that he shortly afterward published his memoir on diffraction. He and Arago then together made the experiments on interference, by which a theoretical explanation of polarization was obtained; but Arago, heartily with him at the beginning, was not able to follow him in all his conclusions, and left to Fresnel the honor of explaining the experiments which he had himself performed.
Oersted having discovered the power of the voltaic current to produce deviation of the magnet, and having thence deduced the theory of the relationship of magnetism and electricity, Arago took up his experiments. With a conductor of copper wire and a pile of iron filings, he learned that the current would also generate magnetism. He communicated his discovery to Ampere, and they made, with knitting-needles, those experiments in electro-magnetism which transformed a whole science, and cleared the way for the electric telegraphs, electric lights, electric clocks, and other instruments of to-day. One day an artisan of the engineers brought him a compass, which was nearly inert in its copper box, but lively enough in action when taken out of it. Experimenting with this apparatus to discover the cause of the compass's inaction, he discovered the magnetism of rotation—a discovery which Faraday complemented by showing how induction-currents are created in the copper.
The observation of a beautiful aurora borealis in 1817 gave Arago opportunity to verify the fact, which had already been observed and remarked upon, that the bands of light and the arch bore a relation to the magnetic meridian; to this he added the new observation that the magnetic needle was disturbed during the whole time of the preva-
