CHAP. V., 6.] 



OPTICS. M. BIOT MR AIRY. 



119 



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- 

 coverer. (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 



quartz), and negative (as calcareous spar). In the 

 former the extraordinary wave is a prolata 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. 



(546.) 



6. Mr AIRY, Sir WILLIAM R. HAMILTON, and Professors LLOYD and MACCULLAGH. Confirma- 

 tion of FresneVs Theory Investigation of the Wave Surface completed ; Conical Refraction. 

 M. CAIJCHY. Mechanical Theory of Elastic Media, and of Ordinary and Metallic 

 Reflection; M. Jamin. Theory of Dispersion ; Professor Powell. 



(547.) It would not be possible, in one short section, to 

 rogress of d o justice to the various improvements and additions 

 itory the-" wn ^ cn ^ e undulatory theory of light the joint crea- 

 my since tion of Huygens, Young, and Fresnel has received 

 resacl. 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. 



(548.) Looking at the history generally, we find one cu- 

 eculiari- r i O us 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 



es in its 

 istory. 



belonged to Great Britain, 1 a few of the most conspi- 

 cuous of whom are named at the head of this section. 

 The attention of the British public was forcibly 

 called to the theory of Young and Fresnel, by an 

 able treatise on Light, contributed by Sir J n n 

 Herschel in 1827 to the Encyclopaedia Metropolitans 

 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 AIRY (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, 5 12). The writings of Mr Airy 

 and of Sir John Herschel have continued to be the 



(549.) 

 r J - HPI 

 * n 



1 M. Moigno (a Frenchman), writing in 1847, laments that France was then perhaps the only country in which the experiment 

 of " conical refraction" (the triumph of Fresnel's theory to be presently mentioned) had never been repeated. 



