238 



Article VI. 

 Memoir on Double Refraction*. By M. A. FREsxELf- 



[From the Memoires de I'Academie Royale des Sciences de I'lnstilvt de France, 

 torn. vii. 1827.] 



Introduction. 



IlUYGENSj guided by an hypothesis founded on the theory of 

 waves, was the first to recognise the true laws of double refrac- 

 tion in uni-axal crystals. This discovery was perhaps more dif- 

 ficult to make than any of Newton's on the subject of light ; and 

 what seems to prove this is, that here Newton, after fruitless 

 attempts to discover the truth, fell into error. When we con- 

 sider how greatly his curiosity must have been excited by the 

 phenomenon of double refraction, we cannot suppose that he 

 gave less attention to it than to other optical pha^nomena, and 

 one is necessarily surprised at seeing him substitute a false rule 

 for the construction of Huygens, as accurate as it was elegant; 

 a construction with which he was no doubt acquainted, because 

 he quotes his treatise on Light. But what appears still more 

 inconceivable is, that the accuracy of Huygens's law was unac- 

 knowledged for more than a hundred years, although it was 

 supported by the experimental verifications of this great man, as 

 remarkable perhaps for his good faith and modesty, as for his 

 rare sagacity. If we ventured to offer an explanation of this 

 singular trait in the history of science, we should say that the 

 considerations drawn from the theory of waves which had guided 

 Huygens, led probably the partisans of the emission system to 

 suppose that he could never have arrived at the truth by a false 



* The Editor is indebted to Alfred W. Hobsoii, B.A., St. John's College, 

 Cambr'dge, for the translation of this memoir. 



-f- The three memoirs, the substance of which is comprised in the present 

 one, were successively presented to the Institute on the 26th Nov. 1821, the 

 22nd Jan. 1822, and 22nd April of the same year. In uniting them, the 

 arrangement of the matter has been changed, and considerable sujjpressioiis 

 made; but nothing essential has been added to the new facts and theoretical 

 views which they contained. To the latter only have been given some deve- 

 lopments necessary for their comprehension; and it has been doenu'd useful to 

 insert in this memoir a complete demonstration of the transversal direction of 

 the luminous vibrations, because on this point depends the theory of pola'-iza- 

 tion and of double refraction. This demonstration has already been published 

 in the Bulletin de la Socicie Philomatique for October 1824. 



