Spectroscopic astronomy had become a distinct and
acknowledged branch of the science, possessing a large literature of
its own, and observatories specially devoted to it. The more recent
discovery of the gelatine dry plate had given a further great impetus
to this modern side of astronomy, and had opened a pathway into the
unknown of which even an enthusiast thirty years ago would scarcely
have dared to dream.
HERSCHEL'S THEORY.
It was now some thirty years since the spectroscope gave us for the
first time certain knowledge of the nature of the heavenly bodies, and
revealed the fundamental fact that terrestrial matter is not peculiar
to the solar system, but is common to all the stars which are visible
to us. Professor Rowland had since shown us that if the whole earth
were heated to the temperature of the sun, its spectrum would resemble
very closely the solar spectrum. In the nebulae, the elder Herschel saw
portions of the fiery mist or "shining fluid," out of which the
heavens and the earth had been slowly fashioned. For a time this view
of the nebulae gave place to that which regarded them as external
galaxies--cosmical "sand heaps," too remote to be resolved into
separate stars, though, indeed, in 1858, Mr.
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