by experiment. By a proper arrangement, as we shall
afterwards learn, we may abolish reflection from one of the surfaces
of the film, and when this is done the rings vanish altogether.

Rings of feeble intensity are also formed by _transmitted_ light.
These are referred by the undulatory theory to the interference of
waves which have passed _directly_ through the film, with others which
have suffered _two_ reflections within the film, and are thus
completely accounted for.

Sec. 10. _The Diffraction of Light_.

Newton's espousal of the Emission Theory is said to have retarded
scientific discovery. It might, however, be questioned whether, in the
long run, the errors of great men have not really their effect in
rendering intellectual progress rhythmical, instead of permitting it
to remain uniform, the 'retardation' in each case being the prelude to
a more impetuous advance. It is confusion and stagnation, rather than
error, that we ought to avoid. Thus, though the undulatory theory was
held back for a time, it gathered strength in the interval, and its
development within the last half century has been so rapid and
triumphant as to leave no rival in the field. We have now to turn to
the investigation of new classes of phenomena, of which it alone can
render a satisfactory account.

Newton, who was familiar with the idea of an ether, and who introduced
it in some of his speculations, objected, as already stated, that if
light consisted of waves shadows could not exist; for that the waves
would bend round the edges of opaque bodies and agitate the ether
behind them. He was right in affirming that this bending ought to
occur, but wrong in supposing that it does not occur. The bending is
real, though in all ordinary cases it is masked by the action of
interference. This inflection of the light receives the name of
_Diffraction_.

To study the phenomena of diffraction it is necessary that our source
of light should be a physical point, or a fine line; for when a
luminous surface is employed, the waves issuing from different points
of the surface obscure and neutralize each other. A _point_ of light
of high intensity is obtained by admitting the parallel rays of the
sun through an aperture in a window-shutter, and concentrating the
beam by a lens of short focus. The small solar image at the focus
constitutes a suitable point of light. The image of the sun formed on
the convex surface of a glass bead, or of a watch-glas