CHAP. V.. 2.] 



OPTICS. MALUS. 



103 



mng's adding, that in private life Dr Young; was exemplary ; 



rsonal endued with warm affections, philosophic moderation, 

 ' and high moral and religious principles. His office 

 as secretary to the Board of Longitude (the only 

 public promotion he received), was attended not only 

 with immense labour in editing the Nautical Al- 

 manac, but with vexatious contentions, which in his, 

 as in so many other cases, tended to diminish his 

 usefulness and even shorten his life. To the petty 

 persecutions with which he was assailed, it was owing 

 that the health which the unbroken study of fifty 

 years had not impaired, at length gave way, and he 



d death. <ij e <i yet j n ^ p r i me o f intellect, the 10th May 

 1829, within a few months of his honoured asso- 

 ciates and friends, Wollaston and Davy. He had 

 been elected two years previously one of the eight 

 foreign associates of the Academy of Sciences of 

 Paris. 



c 



Dr Young's philosophical character approached in (473.) 

 many important particulars to that of Newton. 

 much of the inventive fire of Davy, and of the rea- 

 soning sagacity of Wollaston, he combined an amount 

 of acquired learning, and a versatility in its applica- 

 tion, far superior to both. We do not ascribe to him 

 an intuitive insight so rapid and almost divine as 

 that which distinguished the author of the Principia 

 above all other men, nor had Young the same strictly 

 mathematical ability ; but like Newton, whatever he 

 did was practical and sound ; nothing was done for 

 show, nothing omitted through haste. " The power 

 of patient thought" was the lever with which he 

 moved the world. His self-confidence was great but 

 unobtrusive. He attained, as he himself said, all the 

 main objects to which he had looked forward in life, 

 " such fame as he valued, and such acquirements as 

 he might think to deserve it." 



2. MALUS. Discovery of the Polarization of Light Tyy Reflection. Early History of Double 



Refraction and Polarization. 



(474.) 

 ilus the 

 lariza- 

 mof 

 ht. 



(475.) 

 itly his- 

 ry of 

 uble re- 

 iction 

 lygens' 



ETIENNE Louis MALUS was born at Paris on the 

 23d July 1775, and died on the 24th February 1812, 

 after a too brief but brilliant career. His principal 

 discovery, that of the polarization of light by reflec- 

 tion, is so intimately connected, both historically and 

 by the nature of the case, with double refraction, that 

 I shall briefly sum up the scanty progress of that 

 singular subject previous to his time. 



It was known to Bartholin of Copenhagen, about 

 1669, that Iceland or calcareous-spar has the pro- 

 perty of dividing a ray of light, which falls upon it 

 in almost any direction, into two; one of which is 

 refracted according to the usual law, but the other 

 in an extraordinary manner, which was first analyzed 

 by Huygens a problem of great difficulty, in which 

 Newton not only failed, but he also erred in con- 

 tinuing to pronounce Huygens' solution false. The 

 solution was this, that there is one direction in the 

 crystal parallel to which both the rays (called the 

 ordinary and the extraordinary') move in a similar 

 and uniform manner. In other directions their pro- 

 pagation may be expressed by considering the ordi- 

 nary ray within the crystal to be due to a spherical 

 wave (the centre of which coincides with the point 

 of incidence), whilst the extraordinary ray corre- 

 sponds to a flattened spheroidal wave concentric with 

 the former, and having its axis coincident with a 

 diameter of the sphere, and parallel to the minera- 

 logical axis of the crystal. Both rays, on the Huy- 

 genian hypothesis, move slower than in air, but the 

 extraordinary ray everywhere faster than the ordi- 

 nary ray, excepting only in the axial direction. A 

 perfectly plain though necessarily complex construc- 

 tion was given by Huygens for the purpose of tracing 

 both rays in the course of their refraction, founded 

 on this idea. 



Newton's opposition to Huygens' law as a state- (476.) 

 ment of fact left it for more than a century under 

 partial doubt. Haiiy is stated to have verified it, or ton ; 

 at least to have shown that it approached nearer to 

 the truth than Newton's ; but Dr Wollaston first re- 

 established it in 1802 by conclusive experiments, 

 which, however, he found it impossible to connect by 

 a law until the previous generalization of Huygens 

 had been pointed out to him, most probably by Dr 

 Yoimg. 



It was several years later that Malus directed his (477.) 

 attention to the subject, unaware of what had been ^ * g lso b ? 

 accomplished by Wollaston. He had returned in 

 1801 from the unfortunate French expedition to 

 Egypt, where he was engaged as an officer of en- 

 gineers, and had ruined his health through fatigue 

 and the insalubrity of the climate. He was an ac- 

 complished mathematician, having acted as professor 

 both at the Polytechnic School and that of Metz, 

 and was of course a member of the Institute of Cairo. 

 On his return to France, during the intervals of his 

 military duties, he occupied himself in the composi- 

 tion of an elaborate analytical treatise on op tics, which 

 had already occupied his attention in Egypt. This 

 led him to the subject of double refraction, and he 

 verified by numerous experiments the accuracy of 

 Huygens' law. La Place wrote a paper on the mathe- 

 matical law of the velocity of the extraordinary ray, 

 in which he introduced the idea of a repulsive force 

 emanating from the axis of the crystal ; but it may 

 be truly affirmed that the notion of a spheroidal un- 

 dulation so happily introduced by Huygens is the 

 only one which really fits the case ; and by the very 

 impossibility of expressing the facts intelligibly with- 

 out it, gives an undisputed advantage to that theory. 



A prize having been proposed by the Academy of (478.) 



