(488.) 
Transverse 
vibrations 
—Young 
and Fres- 
nel, 
904 
using it. Such Fresnel possessed, though he always 
refers with great modesty to his limited facility of em- 
ploying the higher mathematics. 
Transverse Vibrations— Young and Fresnel._— 
Considerable obscurity hangs over the first publi- 
cation of this important discovery. A clear and 
impartial abstract of the facts will be found in 
the second volume of Dr Whewell’s History of the 
Inductive Sciences, and some further documentary 
evidence, including interesting letters which passed 
between Young and Fresnel, have more recently 
been published in the Life and Works of the for- 
mer, edited by Dr Peacock. ‘The difficulty of appor- 
tioning the credit between Young and Fresnel partly 
arises from the unfortunate system of imperfect pub- 
lication, or non-publication, adopted on professional 
grounds by the former, and partly from the grievous 
delays imposed upon the latter by the opposition with 
which his opinions and experiments were received at 
the French Academy of Sciences. This continued to 
the very close of Fresnel’s career. His greatest work 
was not published in the Memoirs of the Institute 
until six years after date; another was mislaid for 
above twenty years, and even the hardy friendship of 
Arago sometimes almost recoiled before the storm of 
opposition which the novelties of his associate were 
sure to excite in the minds of the dominant mathe- 
matical section. It is quite impossible to say pre- 
cisely at what period Young first imagined that the 
differences of oppositely polarized rays of light might 
be explained by perpendicularity in the directions of 
vibration of the ethereal molecules, which he compared 
to the vibrations of a cord in which the elementary 
movements are at right angles to the direction of wave- 
propagation. It seems evident that Young was not 
possessed of this idea in 1814, when he partly explained 
depolarization in a few pages of an article in the 
Quarterly Review. It is equally certain that he an- 
nounced it to Arago (with whom he became person- 
ally acquainted in 1816) in January, 1817; and that 
he then speaks of it as an idea which apparently had 
recently occurred to him, most probably since their 
interview. Arago and Fresnel had already, in 1816, 
made experiments demonstrating that rays oppositely 
polarized do not produce dark bands by their inter- 
ference, a memorable discovery, requiring very great 
nicety for its satisfactory proof, which, however, 
was completely attained. It was this observa- 
tion which (naturally) gave Fresnel the first idea 
of transverse vibrations, and it is much more than 
probable that Young worked out a similar solution 
of the great problem, in consequence of the account 
of these experiments which he received from Arago 
in the summer of 1816.7 Be this as it may, Young 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI, 
and Fresnel unquestionably imagined the theory se- 
parately, but Young first announced it, Fresnel being 
discouraged by the doubts of Arago, and by his 
awe of the Institute. As clearly, the experiment of 
non-interference was the first which gave a colour to 
so bold an assumption, and in the details of its ap- 
plication to double refraction Fresnel had the undi- 
vided merit. What is not least worthy of notice in 
the affair, is that neither of the amiable rivals (Young 
and Fresnel) ever published a word in disparagement 
of the other, nor a single unfriendly reclamation of 
priority. 
The doctrine of transverse vibrations being allowed, 489.) 
its applications and severest tests were twofold, 1st, Applied to 
To the phenomena of ordinary reflection and refrac- o¢ light. 
tion including the polarization produced by these ope- 
rations; and 2dly,to double refraction and the univer- 
sally concomitant polarization. In these bold specu- 
lations and laborious inductions, Fresnel was nearly 
alone. Young did not appear as a competitor; even 
his friend Arago, though sympathizing with and 
proud of his success, was not associated with him, 
Laws of Reflection and Refraction.—With re- 
(490.) 
gard to the reflection and refraction of light, its in- Lavi 
tensity has to be defined, and also its condition as to and refrac 
polarization. The fundamental laws of the direction tion— 
of the rays are not affected by this theory. Rigorous Fresnel 
mathematicians who then doubted the possibility 
of transverse vibrations having more than a transi- 
tory existence, if they existed at all, could not be ex- 
pected to supply the theory of their reflection and re- 
fraction at the bounding surfaces of different media. 
Fresnel, however, guided by probable mechanical 
analogies, with an intuitive insight worthy of Newton 
himself, gave a formula for the intensity of reflected 
transverse vibrations, both when the plane of vibration 
of the molecules is in the plane of reflection, and when 
it is perpendicular to it; and he conceived common 
light to act as if equally composed of both sets of 
vibrations. His formule embrace the non-reflection 
of polarized light at the critical angle, under the 
circumstances explained in the last section, It is a 
most remarkable fact that these inferences by Fresnel 
as to the numerical relations of the intensity of the re- 
flected to the incident light through all angles of inci- 
dence, anticipated almost every trustworthy photo- 
metrical measure; and from their singular though 
indirect accordance with many phenomena, they have 
been generally accepted as an expression of a natural 
law of great complexity, even by those who were not 
favourable to the theoretical views on which they are 
based. 
The modifications of the state of polarization of 
light which takes place by reflection, was equally 
1 “T have also been reflecting on the possibility of giving an imperfect explanation of the affection of light which constitutes 
polarization, without departing from the genuine doctrine of undulation.” He then refers to “a transverse vibration propagated 
in the direction of the radius, the motion of the particles being in a certain constant direction with respect to that radius; and 
this,” he adds, “ is a polarization.” —Young’s Miscell. Works, vol. i., p. 883. 
2 Peacock’s Life of Young, p. 390. 
(491 ) 
