, which may stand for Newton's image of the
sun. Causing the beam (from the aperture L, fig. 7) which produces the
disk to pass through a lens (E), we form a sharp image of the
aperture. Placing in the track of the beam a prism (P), we obtain
Newton's coloured image, with its red and violet ends, which he called
a _spectrum_. Newton divided the spectrum into seven parts--red,
orange, yellow, green, blue, indigo, violet; which are commonly called
the seven primary or prismatic colours. The drawing out of the white
light into its constituent colours is called _dispersion_.

[Illustration: Fig. 7.]

This was the first _analysis_ of solar light by Newton; but the
scientific mind is fond of verification, and never neglects it where
it is possible. Newton completed his proof by _synthesis_ in this way:
The spectrum now before you is produced by a glass prism. Causing the
decomposed beam to pass through a second similar prism, but so placed
that the colours are refracted back and reblended, the perfectly white
luminous disk is restored.

[Illustration: Fig. 8.]

In this case, refraction and dispersion are simultaneously abolished.
Are they always so? Can we have the one without the other? It was
Newton's conclusion that we could not. Here he erred, and his error,
which he maintained to the end of his life, retarded the progress of
optical discovery. Dollond subsequently proved that by combining two
different kinds of glass, the colours can be extinguished, still
leaving a residue of refraction, and he employed this residue in the
construction of achromatic lenses--lenses yielding no colour--which
Newton thought an impossibility. By setting a water-prism--water
contained in a wedge-shaped vessel with glass sides (B, fig. 8)--in
opposition to a wedge of glass (to the right of B), this point can be
illustrated before you. We have first of all the position (dotted) of
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 u