a satisfactory and correct elucidation by the aid of the undulatory theory of light. At the time Arago approached this question, only a few of the facts hearing upon the undulatory theory had been determined, while the most of them were unknown, or hardly suspected. He had to begin by finding them out and classifying them, and so bringing himself gradually nearer to the primary ideas. "The work resembled the play of guessing at words, which all the company know but the person who has to find the word. The inquirer has to ply Nature with methodical questions, numerous and close, to extract her secret from her. No one was better suited to the performance of such a part than Arago; no one more obedient to experiment, no one more systematically rebellious to preconceived theories. He began by studying how natural matter becomes polarized, and found that it is when it is divided into two parts. If there is polarized light in one of these parts, an exactly equal quantity of it will be found in the other, both vibrating in perpendicular planes. This mode of division forms a physical law which is still known as Arago's law." From this law Arago drew two practical results. The first one is applicable to lakes and seas, the surface of which divides the light rays into two parts—the reflected part, which takes the color of the sky and vibrates horizontally; and a part which, having penetrated to the interior and having vertical vibrations, is returned to us with the color of the water. "Both parts are mingled, but a double-refracting crystal separates them, and we see in one of the images the reflected sky, and in the other the bottom of the lake and all that it contains." The second result is that the sun, reflecting only natural or unpolarized light, is a flame, an incandescent gas, and not an incandescent solid.
Arago next published his discovery of the phenomena of rotatory polarization, with the production of complementary colors, varying in properties according to the crystalline medium through which they are viewed. One of his experiments was applied to the edification of the public by the optician Soleil, who devised various fanciful designs on laminæ of gypsum, which, colorless in natural light, were transformed, under the working of the polariscope, into polychrome images having the most beautiful appearance. One of the favorite designs was the word "Arago" surrounded by a laurel-wreath.
"Rarely," says M. Jamin, "has an inventor ever reached the limits of his discovery. He looks for its consequences where they are not, he goes astray in the labyrinth where no thread guides him, he passes by the truth without perceiving it, and leaves to his successors to reap where he has sown. Like so many others before him, Arago left the great work he had labored at without completing it. He was endowed with unequaled clairvoyance, and divined discoveries before making them; but he had no patience for details: he opened mines without working them out, and began labors without pursuing them. His
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