110 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



tary to the Paris Observatory. He pursued with M. 

 Biot experiments on the refraction of the gases, and 

 in 1806 the two young philosophers were despatched 

 to the south of France and to Spain to continue the 

 triangulation intercepted by the death of Mechain. 1 

 Geodetical The next three years were spent by Arago in a series 

 observa- O f voluntary and involuntary journeys, perils by land 

 and sea, from robbers, and from the Spanish govern- 

 ment and populace, such as have been rarely equalled, 

 perhaps never in the pursuit of science. The decla- 

 ration of war against France rendered his stay either 

 in Valentia or in the Balearic Isles impossible, and 

 he was conveyed in disguise from Majorca to Algiers, 

 whence he twice essayed to reach Marseilles, but 

 was once driven back to Africa by a storm, once 

 made prisoner by a Spanish corsair. After great 

 suffering, he at length reached France in July 1809, 

 carrying with him the precious record of his geodeti- 

 cal operations. From this time his promotion was 

 assured, and his life became tranquil and inactive, al- 

 though the deep attachment which he formed with 

 Baron Humboldt immediately on his return to France 

 would probably have induced him to accompany that 

 enterprising traveller to Central Asia, had that jour- 

 ney ever been accomplished. At the early age of 23 

 Arago attained the position of Member of the Insti- 

 tute, and was again attached to the Paris Observa- 

 tory, of which at a later period he became director. 

 He took a very active share in the proceedings of the 

 Academy of Sciences, and became one of its secre- 

 taries in 1830. 



(502.) From the period of his election to the Institute, 



Arago's Arago's career was destitute of stirring incidents, but 

 career ^ was ' ^ rom nrs * * ^ as ^' devoted chiefly to science. It is 

 certain, however, that he was deficient in that power 

 of continuous application, to which alone great dis- 

 coveries are commonly due. Full of ingenious, origi- 

 nal, and even profound conceptions, he shunned the 

 labour of realizing them. His appointment to the 

 charge of the Observatory of Paris was perhaps unfor- 

 tunate. Well versed in the theory of astronomy, the 

 minute drudgery of observation and the control of 

 numerous assistants, was altogether uncongenial to 

 him. It was a duty imperfectly fulfilled, to say the 

 least, for 40 years: and it is needless to add how much 

 the consciousness of habitual neglect of a duty deadens 

 the faculty of useful application to anything else. 

 /503.) If the science of astronomy then owes little to. 

 Arago, beyond the part which he took in geodetical 



observation, what are we to consider his chief claims His optica 

 to a place in this history 1 I have no hesitation in labours of 

 saying that they are to be found in connection with ^.^ 

 the discoveries and labours of his attached friends, 

 Malus and Fresnel, and therefore we group them to- 

 gether in this chapter. Arago not only himself made 

 some important optical discoveries in 1811 and the 

 following years, but he was instrumental, as we have 

 seen, in a very important degree, in calling forth the 

 genius of Fresnel, and in obtaining a public recogni- 

 tion of the labours of Young ; a service not the less 

 worthy of note because of its eminently disinterested 

 character. The undulatory theory of light, one of 

 the greatest triumphs, if not the greatest, of our age, 

 stands where it does in no slight degree through 

 the instrumentality of Arago. 



One of his most considerable discoveries was that (504.) 

 of the colours which crystallized bodies develop in Colours of 

 white light polarized before incidence on the crystal, r y s . ta 

 and afterwards transmitted through a rhomb of calc- polarized 

 spar. These colours, by their order, the singular man- light. 

 ner of their occurrence and disappearance, and in cer- 

 tain cases by their extraordinary and beautiful forms, 

 offered a problem at once the most attractive, the 

 most definitely marked, and the most seemingly in- 

 explicable which had been met with in optics for much 

 more than a century. They exemplified a new mode 

 of analyzing light, evidently connected with the mole- 

 cular forces concerned in crystallization; and for their 

 display it was necessary that light should be in that 

 peculiar and yet mysterious state called polarized. 



The substances he employed were principally sele- (505.) 

 nite, rock-crystal, and mica. When plates of the phenomer 

 first and last of these minerals, formed by their natu- zati o* 

 ral cleavage, are placed in a beam of polarized light, 

 and the light transmitted by them is then analyzed, 

 by being passed through a doubly-refracting prism or 

 thrown on a screen after reflection at the critical angle 

 from glass, splendid colours are the result. These 

 colours vary with the thickness of the plate, with its in- 

 clination to the incident light, and, whatis most remark- 

 able, they vary in intensity bymerely turning the plate 

 of selenite round in its own plane. When only this last 

 motion is made, there are two positions of the plate 

 where no colour results, the light passing through un- 

 changed; and these positions are at right angles to one 

 another. At all intermediate angles colours appear, the 

 light is said to be depolarized, and this depolarization 

 is most complete when the plate is moved 45 from 



the difficulties inseparable from the biographical system which I have adopted. My intention was to have thrown together the 

 labours of Malus, Fresnel, and Arago into one section. But having written the different portions separately, there seemed so 

 much precision and facility of explanation to be derived from treating of them consecutively, that I sacrificed, in some degree, 

 the biographical principle to that of systematic classification ; placing under the name of Malus what referred to the empirical 

 laws of double refraction ; under that of Fresnel the doctrine of transverse vibration (though mainly due to Young) ; and under 

 Arago the discoveries of Young, Fresnel, Biot, and others, relative to the great subject of chromatic polarization, to which he 

 gave the first impulse. The establishment of the undulatory theory, principally due to Young, Fresnel, and Arago, I have con- 

 sidered as deserving of a more detailed and systematic treatment than almost any other of the numerous discoveries of which I 

 have to speak in this Dissertation. I may add, that the biography of Arago not appearing in its alphabetical place in the En- 

 cyclopaedia, M. Arago being still alive at the date of the publication of that part of the work, it has been incumbent upon me to 

 enter into more details than I should otherwise have done. * See Chapter III., Art. (166) of this Dissertation. 



