regulated slit.

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. To observe this you place
a Bunsen burner opposite the slit of the spectroscope, and introduce
into its colourless flame on the end of a platinum wire a little of a
volatile salt of the metal or element to be examined. The flame of the
lamp itself is often coloured with a distinctiveness that is sufficient
for a judgment to be made with the aid of the naked eye alone, as to the
metal or element present. Thus soda and its salts give a yellow flame,
which is absolutely yellow or monochromatic, and if you look through
your prism or spectroscope at it, you do not see a coloured rainbow band
or spectrum, as with daylight or gaslight, but only one yellow double
line, just where the yellow would have been if the whole spectrum had
been represented. I think it is now plain that for the sake of
observations and exact discrimination, it is necessary to map out our
spectrum, and accordingly, in one of the tubes, the third, the
spectroscope is provided with a graduated scale, so adjusted that when
we look at the spectrum we also see the graduations of the scale, and so
our spectrum is mapped; the lines marked out and named with the large
and small letters of the alphabet, are certain of the prominent
Fraunhofer's lines (see A, B, C, a, d, etc., Fig. 16). We speak, for
example, of the