SKETCH OF M. ARAGO. 263 



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 ex- 

 cited 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 per- 

 severance that finishes what is begun. ... Of theoretical ideas which 

 include a whole science in a few general hypotheses, and leave an in- 

 effaceable trace, he produced none, but sometimes repelled them, even 

 when his own experiments led others to them." Biot, his former col- 

 laborator, 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 undula- 

 tory 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 after- 

 ward published his memoir on diffraction. He and Arago then to- 

 gether 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 conclu- 

 sions, 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 pro- 

 duce 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 knit- 

 ting-needles, those experiments in electro - magnetism which trans- 

 formed 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- 



