from Bradtey to Fresnel. 121 



tion of the apparatus relative to the direction of the earth's 

 motion. E. Mascart* in 1872 discussed experimentally the 

 question of the effect of motion of the source or recipient of 

 light in all its bearings, and showed that the light of the sun and 

 that derived from artificial sources are alike incapable of revealing 

 by diffraction-phenomena the translatory motion of the earth. 



The greatest problem now confronting the investigators of 

 light was to reconcile the facts of polarization with the principles 

 of the wave-theory. Young had long been pondering over this, 

 but had hitherto been baffled by it. In 1816 he received a 

 visit from Arago, who told him of a new experimental result 

 which he and Fresnel had lately obtained! namely, that two 

 pencils of light, polarized in planes at right angles, do not 

 interfere with each other under circumstances in which ordinary 

 light shows interference-phenomena, but always give by their 

 reunion the same intensity of light, whatever be their difference 

 of path. 



Arago had not long left him when Young, reflecting on the 

 new experiment, discovered the long-sought key to the mystery : 

 it consisted in the very alternative which Bernoulli had rejected 

 eighty years before, of supposing that the vibrations of light are 

 executed at right angles to the direction of propagation. 



Young's ideas were first embodied in a letter to Arago,J 

 dated Jan. 12, 1817. "I have been reflecting," he wrote, " on the 

 possibility of giving an imperfect explanation of the affection 

 of light which constitutes polarization, without departing from 

 the genuine doctrine of undulations. It is a principle in this 

 theory, that all undulations are simply propagated through 

 homogeneous mediums in concentric spherical surfaces like the 



Ann. de 1'Ecole Noemale, (2) i, p. 157. 



t It was not published until 1819, in Annales de Chimie, x ; Fresnel's (Euvres, 

 i, p. 509. By means of this result, Fresnel was able to give a complete explana- 

 tion of a class of phenomena which Arago had discovered in 1811, viz. that when 

 polarized light is transmitted through thin plates of sulphate of lime or mica, and 

 afterwards analysed by a prism of Iceland spar, beautiful complementary colours 

 are displayed. Young had shown that these effects are due essentially to inter- 

 ference, hut had not made clear the part played by polarization. 



J Young's JTorks, i., p. 380. 



