262 THE POPULAR SCIENCE MONTHLY. 



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 deter- 

 mined, 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. lie 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 divis- 

 ion 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 unpo- 

 larized 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 laminae of gypsum, which, colorless in natural light, were trans- 

 formed, under the working of the polariscopc, 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 lias sown. Like so many others before him, Arago left the 

 great work he had labored at without completing it. He was endowed 

 with tmequaled 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 



