s blackened
within, though less intense, will also answer. An intense _line_ of
light is obtained by admitting the sunlight through a slit and sending
it through a strong cylindrical lens. The slice of light is contracted
to a physical line at the focus of the lens. A glass tube blackened
within and placed in the light, reflects from its surface a luminous
line which, though less intense, also answers the purpose.

In the experiment now to be described a vertical slit of variable
width is placed in front of the electric lamp, and this slit is looked
at from a distance through another vertical slit, also of variable
aperture, and held in the hand.

The light of the lamp being, in the first place, rendered
monochromatic by placing a pure red glass in front of the slit, when
the eye is placed in the straight line drawn through both slits an
extraordinary appearance (shown in fig. 15) is observed. Firstly, the
slit in front of the lamp is seen as a vivid rectangle of light; but
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 illu