Malus, in 1808, was led by a casual observation of the light of the
setting sun, reflected from the windows of the Palais de Luxembourg,
at Paris, to investigate more thoroughly the phenomena of double
refraction, of ordinary and of chromatic polarisation, of interference
and of diffraction of light. Among his results may be reckoned the
means of distinguishing between direct and reflected light; the power
of penetrating, as it were, into the constitution of the body of
the sun and of its luminous envelopes; of measuring the pressure of
atmospheric strata, and even the smallest amount of water they contain;
of ascertaining the depths of the ocean and its rocks by means of
a tourmaline plate; and in accordance with Newton’s prediction, of
comparing the chemical composition of several substances with their
optical effects.
Arago, in a letter to Humboldt, states that by the aid of his
polariscope, he discovered, before 1820, that the light of all
terrestrial objects in a state of incandescence, whether they be
solid or liquid, is natural, so long as it emanates from the object
in perpendicular rays. On the other hand, if such light emanate
at an acute angle, it presents manifest proofs of polarisation.
This led M. Arago to the remarkable conclusion, that light is not
generated on the surface of bodies only, but that some portion is
actually engendered within the substance itself, even in the case
of platinum.
A ray of light which reaches our eyes after traversing many millions
of miles, from, the remotest regions of heaven, announces, as it were
of itself, in the polariscope, whether it is reflected or refracted,
whether it emanates from a solid or fluid or gaseous body; it announces
even the degree of its intensity.--_Humboldt’s Cosmos_, vols. i. and ii.