The principle of interference, as just stated, applies to the waves of
light as it does to the waves of water and the waves of sound. And the
conditions of interference are the same in all three. If two series of
light-waves of the same length start at the same moment from a common
origin (say A, fig. 11), crest coincides with crest, sinus with sinus,
and the two systems blend together to a single system (A _m_ _n_) of
double amplitude. If both series start at the same moment, one of them
being, at starting, a whole wavelength in advance of the other, they
also add themselves together, and we have an augmented luminous
effect. The same occurs when the one system of waves is any _even_
number of semi-undulations in advance of the other. But if the one
system be half a wave-length (as at A' _a_', fig. 12), or any _odd_
number of half wavelengths, in advance, then the crests of the one
fall upon the sinuses of the other; the one system, in fact, tends to
_lift_ the particles of ether at the precise places where the other
tends to _depress_ them; hence, through the joint action of these
opposing forces (indicated by the arrows) the light-ether remains
perfectly still. This stillness of the ether is what we call darkness,
which corresponds with a dead level in the case of water.

[Illustration: Fig. 12.]

It was said in our first lecture, with reference to the colours
produced by absorption, that the function of natural bodies is
selective, not creative; that they extinguish certain constituents of
the white solar light, and appear in the colours of the unextinguished
light. It must at once occur to you that, inasmuch as we have in
interference an agency by which light may be self-extinguished, we may
have in it the conditions for the production of colour. But this would
imply that certain constituents are quenched by interference, while