on optics, he clinched the argument
and established his discovery. White light was not simple but compound.
It could be sorted out by a prism into an infinite number of constituent
parts which were differently refracted, and the most striking of which
Newton named violet, indigo, blue, green, yellow, orange, and red.

[Illustration: FIG. 64.--A single constituent of white light, obtained
by the use of perforated screens is capable of no more dispersion.]

At once the true nature of colour became manifest. Colour resided not in
the coloured object as had till now been thought, but in the light which
illuminated it. Red glass for instance adds nothing to sunlight. The
light does not get dyed red by passing through the glass; all that the
red glass does is to stop and absorb a large part of the sunlight; it is
opaque to the larger portion, but it is transparent to that particular
portion which affects our eyes with the sensation of red. The prism acts
like a sieve sorting out the different kinds of light. Coloured media
act like filters, stopping certain kinds but allowing the rest to go
through. Leonardo's and all the ancient doctrines of colour had been
singularly wrong; colour is not in the object but in the light.

Goethe, in his _Farbenlehre_, endeavoured to controvert Newton, and to
reinstate something more like the old views; but his failure was
complete.

Refraction analysed out the various constituents of white light and
displayed them in the form of a series of overlapping images of the
aperture, each of a different colour; this series of images we call a
spectrum, and the operation we now call spectrum analysis. The reason of
the defect of lenses was now plain: it was not so much a defect of the
lens as a defect of light. A lens acts by refraction and brings rays to
a focus. If light be simple it acts well, but if ordinary white light
fall upon a lens, its different constituents have different foci; every
bright object is fringed with colour, and nothing like a clear image can
be obtained.

[Illustration: FIG. 65.--Showing the boundary rays of a parallel beam
passing through a lens.]

A parallel beam passing through a lens becomes conical; but instead of a
single cone it is a sheaf or nest of cones, all having the edge of the
lens as base, but each having a different vertex. The violet cone is
innermost, near the lens, the red cone outermost, while the others lie
between. Beyond the crossing point or focus