resting
observations on the polarization of the light of the sky. To him is also
due the discovery of the power of _rotatory polarization_ exhibited by
quartz, and last of all, among his many contributions to the support of
the undulatory hypothesis, comes the _experimentum crucis_ which he
proposed to carry out for comparing directly the velocity of light in
air and in water or glass. On the emission theory the velocity should be
accelerated by an increase of density in the medium; on the wave theory,
it should be retarded. In 1838 he communicated to the Academy the
details of his apparatus, which utilized the revolving mirrors employed
by Sir C. Wheatstone in 1835 for measuring the velocity of the electric
discharge; but owing to the great care required in the carrying out of
the project, and to the interruption to his labours caused by the
revolution of 1848, it was the spring of 1850 before he was ready to put
his idea to the test; and then his eyesight suddenly gave way. Before
his death, however, the retardation of light in denser media was
demonstrated by the experiments of H.L. Fizeau and J.B.L. Foucault,
which, with improvements in detail, were based on the plan proposed by
him.

Arago's _OEuvres_ were published after his death under the direction
of J.A. Barral, in 17 vols., 8vo, 1854-1862; also separately his
_Astronomie populaire_, in 4 vols.; _Notices biographiques_, in 3
vols.; _Notices scientifiques_, in 5 vols.; _Voyages scientifiques_,
in 1 vol.; _Memoires scientifiques_, in 2 vols.; _Melanges_, in 1
vol.; and _Tables analytiques et documents importants_ (with
portrait), in 1 vol. English translations of the following portions of
his works have appeared:--_Treatise on Comets_, by C. Gold, C.B.
(London, 1833); also translated by Smyth and Grant (London, 1861);
_Hist. eloge of James Watt_, by James Muirhead (London, 1839); also
translated, with notes, by Lord Brougham; _Popular Lectures on
Astronomy_, by Walter Kelly and Rev. L. Tomlinson (London, 1854); also
translated by Dr W.H. Smyth and Prof. R. Grant, 2 vols. (London,
1855); _Arago's Autobiography_, translated by the Rev. Baden Powell
(London, 1855, 1858); _Arago's Meteorological Essays_, with
introduction by Humboldt, translated under the superintendence of
Colonel Sabine (London, 1855), and _Arago's Biographies of Scientific
Men_, translated by Smyth, Powell and Grant, 8vo (London, 1857).

ARAGON, or ARRA