M. Arago has found, by experiments with the polariscope, that the light
of gaseous bodies is natural light when it issues from the burning
surface; although this circumstance does not prevent its subsequent
complete polarisation, if subjected to suitable reflections or
refractions. Hence we obtain _a most simple method of discovering
the nature of the sun_ at a distance of forty millions of leagues.
For if the light emanating from the margin of the sun, and radiating
from the solar substance _at an acute angle_, reach us without having
experienced any sensible reflections or refractions in its passage
to the earth, and if it offer traces of polarisation, the sun must
be _a solid or a liquid body_. But if, on the contrary, the light
emanating from the sun’s margin give no indications of polarisation,
the _incandescent_ portion of the sun must be _gaseous_. It is by means
of such a methodical sequence of observations that we may acquire
exact ideas regarding the physical constitution of the sun.--_Note to
Humboldt’s Cosmos_, vol. iii.