908 
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 
MATHEMATICAL AND PHYSICAL SCIENCE. [Diss, VI. 
observation, what are we to consider his chief claims His optical 
to a place in this history? I have no hesitation in labours of 
saying that they are to be found in connection with Tate a 
Geodetical 
observa- 
tions ; 
to the south of France and to Spain to continue the 
triangulation interrupted by the death of Méchain.1 
The next three years were spent by Arago in a series 
of 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 neverin 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, 
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. 
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- 
of the colours which crystallized bodies develop in Colours of 
white light polarized before incidence on the crystal, sg 
and afterwards transmitted through a rhomb of cale- polarized 
though the deep attachment which he formed with 
Baron Humboldt immediately on his return to France 
* would probably have induced him to accompany that 
(502.) 
Arago’s 
subsequent 
career ; 
(503.) 
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. 
From the period of his election to the Institute, 
Arago’s career was destitute of stirring incidents, but 
was, from first to last, 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. 
If the science of astronomy then owes little to 
Arago, beyond the part which he took in geodetical 
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 formuch 
more thana 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- 
nite, rock-crystal, and mica, When plates of the 
first and last of these minerals, formed by their natu- 
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 tothe incidentlight, and, whatis mostremark- 
able, they vary in intensity by merely 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 allintermediate 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- 
cyclopadia, M. Arago being stillalive 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, 
1 See Chapter IIL, Art. (166) of this Dissertation. 
(505.) 
Phenomena 
of depolar- 
ization. 
