cribed as a mode of distinguishing the various species of
matter by the kind of light proceeding from each. This definition at
once explains how it is that, unlike every other system of chemical
analysis, it has proved available in astronomy. Light, so far as
_quality_ is concerned, ignores distance. No intrinsic change, that we
yet know of, is produced in it by a journey from the farthest bounds of
the visible universe; so that, provided only that in _quantity_ it
remain sufficient for the purpose, its peculiarities can be equally well
studied whether the source of its vibrations be one foot or a hundred
billion miles distant. Now the most obvious distinction between one kind
of light and another resides in colour. But of this distinction the eye
takes cognisance in an aesthetic, not in a scientific sense. It finds
gladness in the "thousand tints" of nature, but can neither analyse nor
define them. Here the refracting prism--or the combination of prisms
known as the "spectroscope"--comes to its aid, teaching it to measure as
well as to perceive. It furnishes, in a word, an accurate scale of
colour. The various rays which, entering the eye together in a confused
crowd, produce a compound impression made up of undistinguishable
elements, are, by the mere passage through a triangular piece of glass,
separated one from the other, and ranged side by side in orderly
succession, so that it becomes possible to tell at a glance what kinds
of light are present, and what absent. Thus, if we could only be assured
that the various chemical substances when made to glow by heat, emit
characteristic rays--rays, that is, occupying a place in the spectrum
reserved for them, and for them _only_--we should at once be in
possession of a mode of identifying such substances with the utmost
readiness and certainty. This assurance, which forms the solid basis of
spectrum analysis, was obtained slowly and with difficulty.

The first to employ the prism in the examination of various flames (for
it is only in a state of vapour that matter emits distinctive light) was
a young Scotchman named Thomas Melvill, who died in 1753, at the age of
twenty-seven. He studied the spectrum of burning spirits, into which
were successively introduced sal ammoniac, potash, alum, nitre, and
sea-salt, and observed the singular predominance, under almost all
circumstances, of a particular shade of yellow light, perfectly definite
in its degree of refrangibility[368]--