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.
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