(547.) 
Cuar. V., § 6.] 
laboriously accurate, One of his papers fills an 
entire volume of the Academy’s Memoirs. Even 
the astronomical hieroglyphics of the Egyptians, and 
the chronology of Chinese eclipses, have drawn from 
his pen learned treatises ; and he has expounded the 
labours and discoveries of his countrymen and others 
with almost as much care and effort as if they had 
been proper to himself. But his subject by predi- 
lection was optics, and here he made his most con- 
siderable discovery, and that which he has followed 
out with most minute industry, namely, the rotatory 
action of fluids, in which he had Seebeck for a co-dis- 
coyerer. (See Art. 512.) He studied the colours of 
crystallized plates with exemplary patience, and as 
we have seen in the preceding section, by his accu- 
rate observations on the law of the tints, prepared 
the way for the theory of transverse vibrations ; but 
his own doctrine of moveable polarization, which he 
imagined to explain them, made no impression on 
the progress of science. He was the first who di- 
vided doubly-refracting crystals into positive (as 
OPTICS.—M. BIOT—MR AIRY. 
917 
quartz), and negative (as calcareous spar), In the 
former the extraordinary wave is a prolate spheroid, 
and inclosed within the ordinary spherical wave ; in 
the latter the spheroid is oblate, and exterior to the 
sphere. He also discovered (very approximately) 
the law regulating the plane of polarization of the 
rays in biaxal crystals. 
M. Biot has for about half a century been an active 
professor and member of the Institute. His researches, 
always marked by precision, are perhaps deficient in 
bold conjecture and happy generalization, They are 
conducted with a mathematical stiffness which allows 
little play to the fancy, and in hypothetical reason- 
ing he rarely indulges. His style is formal yet diffuse, 
and consequently somewhat repulsive to the student. 
His works are consequently not easily read, and have 
contributed less to the progress of knowledge than the 
scrupulous care often evinced in their compilation 
might seem to warrant. Yet the name of Biot will be 
ever associated with devotion to science, and especially 
with the progress of optics in our own day, 
§ 6. Mr Arry, Sir Witr1am R. Hamitton, and Professors Luoyp and Maccuttacu.—Conjirma- 
tion of Fresnel’s Theory—Investigation of the Wave Surface completed ; Conical Refraction. 
—M. Caucnuy. 
Mechanical Theory of Elastic Media, and of Ordinary and Metallic 
Reflection ; M. Jamin.—Theory of Dispersion; Professor Powell. 
It would not be possible, in one short section, to 
Progress of do justice to the various improvements and additions 
the undula- Which the undulatory theory of light—the joint crea- 
latory the- 
ory since 
Fresnel. 
(548.) 
Peculiari- 
ties in its 
history. 
tion of Huygens, Young, and Fresnel—has received 
since the nearly simultaneous decease of the two last- 
named philosophers. But while a vast amount of 
labour and of mathematical and experimental skill 
has been thus expended, of which it would be in vain 
to attempt within our limits to give an account, we may 
pause upon two or three of the more conspicuous re- 
sults of these researches, which, in conformity with 
the plan of this dissertation, may give a tolerable 
idea of their general tendency. 
Looking at the history generally, we find one cu- 
rious peculiarity in the progress of this remarkable 
theory. Its origin in the seventeenth century was 
unattended with sympathy or success. It received 
little support, and was well nigh forgotten for more 
than an hundred years: it was then resumed (we might 
almost say re-invented) in England, but it remained 
unpopular and almost unknown until re-echoed from 
a foreign land; while in France itself the views of 
Fresnel were (with one or two exceptions) as little 
appreciated as those of Young had been in England. 
From this period England became the place of its 
chief development; and with the exception of one 
eminent philosopher, M. Cauchy, its supporters and 
extenders, whether by analysis or experiment, have 
belonged to Great Britain,! a few of the most conspi- 
cuous of whom are named at the head of this section. 
(546.) 
The attention of the British public was forcibly (549. 
called to the theory of Young and Fresnel, by an Sir J. Her- 
able treatise on Light, contributed by Sir John 
Herschel in 1827 to the Encyclopedia Metropolitana. 
The excellent method, lucid explanations, and intel- 
ligent zeal which marked this essay compelled the no- 
tice of men of science, too long deterred from the study 
of the fragmentary and abstruse writings of Young. 
It was followed four years later by a most able and 
precise mathematical exposition of the theory, and its 
application to optical problems, by Mr Arry (now As- 
tronomer Royal), who was then Plumian Professor at 
Cambridge, and who introduced this part of optics 
as a branch of study in that university. Whilst the 
excellent tract on the undulatory theory (published in 
1831 in his Mathematical Tracts) opened up the 
subject in a most accessible form to British mathe- 
maticians, his original papers in the Cambridge 
Transactions confirmed the doctrines of Fresnel by a 
number of new and admirably contrived experiments, 
some in connection with interference, some with po- 
larization, and all were confronted with the rigorous 
results of the mathematical theory. The paper on 
Quartz, and that on the Rainbow, have been already 
referred to (art. 466,512). The writings of Mr Airy 
and of Sir John Herschel have continued to be the 
1M. rf ee (a Frenchman), writing in 1847, laments that France was then perhaps the only country in which the experiment 
of “ conic 
refraction” (the triumph of Fresnel’s theory to be presently mentioned) had never been repeated. 
schel and 
Mr Airy. 
