His apparatus was used at the Paris Observatory in examining the light of
Capella and that of the great comet of 1819. The latter showed polarized,
and therefore reflected light, while the fixed star, as was to be expected,
appeared to be a self-luminous sun.*
[footnote] *On the 3d of July, 1819, Arago made the first attempt to
analyze the light of comets by polarization, on the evening of the sudden
appearance of the great comet. I was present at the Paris Observatory, and
was fully convinced, as were also Matthieu and the late Bouvard of the
dissimilarity in the intensity of the light seen in the polariscope, when
the instrument received cometary light. When it received light from
Capella, which was near the comet, and at an equal altitude, the images were
of equal intensity. On the reappearance of Halley's comet in 1835, the
instrument was altered so as to give, according to Arago's chromatic
polarization, two images of complementary colors (green and red). ('Annales
de Chimie', t. xiii., p. 108; 'Annuaire', 1832, p. 216.) "We must conclude
from these observations," says Arago, "that the cometary light was not
entirely composed of rays having the properties of direct light, there being
light which was reflected specularly or polarized, that is, coming from the
sun.
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