Young's 
personal 
character, 
and death, 
(474.) 
Malus—the 
polariza-~ 
tion of 
light. 
(475.) 
Early his- 
tory of 
double re- 
fraction— 
Huygens’ 
law. 
Cuar. V., § 2.] 
adding, that in private life Dr Young was exemplary ; 
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 
died yet in the prime of 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. 
OPTICS.—MALUS. 
901 
Dr Young’s philosophical character approached in 
many important particulars to that of Newton. With 
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 donot 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 by Reflection—Early History of Double 
Refraction and Polarization. 
Errenne Lovis Matus 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 
erystal 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 erystal 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- 
ment of fact left it for more than a century under 
partial doubt. Haiiy is stated to have verified it, or 
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 
Young. 
It was several years later that Malus directed his 
attention to the subject, unaware of what had been 
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 optics, 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 
(473.) 
Philosophi- 
cal charac- 
ter. 
(476.) 
Confirmed 
by Wollas- 
ton ; 
(477.) 
and also by 
Malus. 
(478.) 
