from Bradley to FremeL 111 



substance ; the spheroid being in the former case oblate and in 

 the latter oblong." Young then proceeds to a formal proof 

 that "an impulse is propagated through every perpendicular 

 section of a lamellar elastic substance in the form of an elliptic 

 undulation." This must be regarded as the beginning of 

 the dynamical theory of light in crystals. It was confirmed 

 in a striking way not long afterwards by Brewster,* who found 

 that compression in one direction causes an isotropic transparent 

 solid to become doubly-refracting. 



Meanwhile, in January, 1808, the French Academy had 

 proposed as the subject for the physical prize in 1810, " To 

 furnish a mathematical theory of double refraction, and to 

 confirm it by experiment." Among those who resolved to 

 compete was Etienne Louis Malus (b. 1775, d. 1812), a colonel 

 of engineers who had seen service with Napoleon's expedition 

 to Egypt. While conducting experiments towards the end of 

 1808 in a house in the Kue des Enfers in Paris, Malus happened 

 to analyse with a rhomb of Iceland spar the light of the setting 

 sun reflected from the window of the Luxembourg, and was 

 surprised to notice that the two images were of very different 

 intensities. Following up this observation, he found that light 

 which had been reflected from glass acquires thereby a modifi- 

 cation similar to that which Huygens had noticed in rays 

 which have experienced double refraction, and which Newton 

 had explained by supposing rays of light to have " sides." This 

 discovery appeared so important that without waiting for the 

 prize competition he communicated it to the Academy in 

 December, 1808, and published it in the following month.f 

 " I have found," he said, " that this singular disposition, 

 which has hitherto been regarded as one of the peculiar effects 

 of double refraction, can be completely impressed on the 

 luminous molecules by all transparent solids and liquids." 

 " For example, light reflected by the surface of water at an 



* Phil. Trans., 1815, p. 60. 



tNouveau Bulletin des Sciences, par la Soc. Philomatique. i (1809), p. 266; 

 Memoires de la Soc. d'Arcueil, ii (1809). 



