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
others are permitted to remain. This is the fact; and it is entirely
due to the difference in the lengths of the waves of light.

Sec. 7. _Colours of thin Films. Observations of Boyle and Hooke_.

This subject may be illustrated by the phenomena which first suggested
the undulatory theory to the mind of Hooke. These are the colours of
thin transparent films of all kinds, known as the _colours of thin
plates_. In this relation no object in the world possesses a deeper
scientific interest than a common soap-bubble. And here let me say
emerges one of the difficulties which the student of pure science
encounters in the presence of 'practical' communities like those of
America and England; it is not to be expected that such communities
can entertain any profound sympathy with labours which seem so far
removed from the domain of practice as are many of the labours of the
man of science. Imagine Dr. Draper spending his days in blowing
soap-bubbles and in studying their colours! Would you show him the
necessary patience, or grant him the necessary support? And yet be it
remembered it was thus that minds like those of Boyle, Newton and
Hooke were occupied; and that on such experiments has been founded a
theory, the issues of which are incalculable. I see no other way for
you, laymen, than to trust the scientific man with the choice of his
inquiries; he stands before the tribunal of his peers, and by their
verdict on