right and left of it is a long series of rectangles, decreasing in
vividness, and separated from each other by intervals of absolute
darkness.

The breadth of these bands is seen to vary with the width of the slit
held before the eye. When the slit is widened the bands become
narrower, and crowd more losely together; when the slit is narrowed,
the individual bands widen and also retreat from each other, leaving
between them wider spaces of darkness than before.

[Illustration: Fig. 15.]

Leaving everything else unchanged, let a blue glass or a solution of
ammonia-sulphate of copper, which gives a very pure blue, be placed in
the path of the light. A series of blue bands is thus obtained,
exactly like the former in all respects save one; the blue rectangles
are _narrower_, and they are _closer together_ than the red ones.

If we employ colours of intermediate refrangibilities, which we may do
by causing the different colours of a spectrum to shine through the
slit, we obtain bands of colour intermediate in width, and occupying
intermediate positions, between those of the red and blue. The aspect
of the bands in red, green, and violet light is represented in fig.
16. When _white light_, therefore, passes through the slit the various
colours are not superposed, and instead of a series of monochromatic
bands, separated from each other by intervals of darkness, we have a
series of coloured spectra placed side by side. When the distant slit
is illuminated by a candle flame, instead of the more intense electric
light, or when a distant platinum wire raised to a white heat by an
electric current is employed, substantially the same effects are