2 from the brightest point of the comet.
The star remained continually visible, and its light was not perceptibly
diminished, while the nucleus of the comet seemed to be almost extinguished
before the radiance of the small star of the ninth or tenth magnitude."
If such an absence of refracting power must be ascribed to the nucleus of a
comet, we can scarcely regard the matter composing comets as a gaseous
fluid. The question here arises whether this absence of refracting power
may not be owing to the extreme tenuity of the fluid; or does the comet
consist of separated particles, constituting a cosmical stratum of clouds,
which, like the clouds of our atmosphere, that exercise no influence on the
p 105
zenith distance of the stars, does not affect the ray of light passing
through it? In the passage of a comet over a star, a more or less
considerable diminution of light has often been observed; but this has been
justly ascribed to the brightness of the ground from which the star seems to
stand forth during the passage of the comet.
The most important and decisive observations that we possess on the nature
and the light of comets are due to Arago's polarization experiments. His
polariscope instructs us regarding the physical constitution of the Sun and
comets, indicating whether a ray that reaches us from a distance of many
millions of miles transmits light directly or by reflection; and if the
former, whther the source of light is a solid, a liquid, or a gaseous body.
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