the unrefracted beam marked upon the screen; then we produce the
narrow water-spectrum (W); finally, by introducing a flint-glass
prism, we refract the beam back, until the colour disappears (at A).
The image of the slit is now _white_; but though the dispersion is
abolished, there remains a very sensible amount of refraction.

This is the place to illustrate another point bearing upon the
instrumental means employed in these lectures. Bodies differ widely
from each other as to their powers of refraction and dispersion. Note
the position of the water-spectrum upon the screen. Altering in no
particular the wedge-shaped vessel, but simply substituting for the
water the transparent bisulphide of carbon, you notice how much higher
the beam is thrown, and how much richer is the display of colour. To
augment the size of our spectrum we here employ (at L) a slit, instead
of a circular aperture.[6]

[Illustration: Fig. 9.]

The synthesis of white light may be effected in three ways, all of
which are worthy of attention: Here, in the first instance, we have a
rich spectrum produced by the decomposition of the beam (from L, fig.
9). One face of the prism (P) is protected by a diaphragm (not shown
in the figure), with a longitudinal slit, through which the beam
passes into the prism. It emerges decomposed at the other side. I
permit the colours to pass through a cylindrical lens (C), which so
squeezes them together as to produce upon the screen a sharply defined
rectangular image of the longitudinal slit. In that image the colours
are reblended, and it is perfectly white. Between the prism and the
cylindrical lens may be seen the colours, tracking themselves through
the dust of the room. Cutting off the more refrangible fringe by a