handelier, or in the stopper of a wine-decanter.

The same effect may be produced when light passes through water. The
Rainbow, which we all know so well, is merely the result of the sunlight
passing through drops of falling rain.

White light is composed of rays of various colours. Red, orange, yellow,
green, blue, indigo, and violet, taken all together, go, in fact, to
make up that effect which we call white.

It is in the course of the _refraction_, or bending of a beam of light,
when it passes in certain conditions through a transparent and denser
medium, such as glass or water, that the constituent rays are sorted out
and spread in a row according to their various colours. This production
of colour takes place usually near the edges of a lens; and, as will be
recollected, proved very obnoxious to the users of the old form of
refracting telescope.

It is, indeed, a strange irony of fate that this very same production
of colour, which so hindered astronomy in the past, should have aided it
in recent years to a remarkable degree. If sunlight, for instance, be
admitted through a narrow slit before it falls upon a glass prism, it
will issue from the latter in the form of a band of variegated colour,
each colour blending insensibly with the next. The colours arrange
themselves always in the order which we have mentioned. This seeming
band is, in reality, an array of countless coloured images of the
original slit ranged side by side; the colour of each image being the
slightest possible shade different from that next to it. This strip of
colour when produced by sunlight is called the "Solar Spectrum" (see
Fig. 9, p. 123). A similar strip, or _spectrum_, will be produced by any
other light; but the appearance of the strip, with regard to
preponderance of particular colours, will depend upon the character of
that light. Electric light and gas light yield spectra not unlike that
of sunlight; but that of gas is less rich in blue and violet than that
of the sun.

The Spectroscope, an instrument devised for the examination of spectra,
is, in its simplest form, composed of a small tube with a narrow slit
and prism at one end, and an eye-piece at the other. If we drop ordinary
table salt into the flame of a gas light, the flame becomes strongly
yellow. If, then, we observe this yellow flame with the spectroscope, we
find that its spectrum consists almost entirely of two bright yellow
transverse lines. Chemically considered o