Now all solid and liquid bodies when raised to a white heat give a
continuous spectrum, one like the prismatic band already described, and
one not interrupted by any dark lines or bands. The rays emitted from
the white-hot substance of the sun have to pass, before reaching our
earth, through the sun's atmosphere, and since the light emitted from
any incandescent body is absorbed on passing through the vapour of that
substance, and since the sun is surrounded by such an atmosphere of the
vapours of various metals and substances, hence we have, on examining
the sun's spectrum, instead of coloured bands or lines only, many dark
ones amongst them, which are called Fraunhofer's lines. Ordinary
incandescent vapours from highly heated substances give discontinuous
spectra, _i.e._ spectra in which the rays of coloured light are quite
limited, and they appear in the spectroscope only as lines of the
breadth of the slit. These are called line-spectra, and every chemical
element possesses in the incandescent gaseous state its own
characteristic lines of certain colour and certain refrangibility, by
means of which that element can be recognised.
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