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 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 his labours you ought to abide.

Whence, then, are derived the colours of the soap-bubble?  Imagine a beam of white light impinging on the bubble.  When it reaches the first surface of the film, a known fraction of the light is reflected back.  But a large portion of the beam enters the film, reaches its second surface, and is again in part reflected.  The waves from the second surface thus turn back and hotly pursue the waves from the first surface.  And, if the thickness of the film be such as to cause the necessary retardation, the two systems of waves interfere with each other, producing augmented or diminished light, as the case may be.