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.


§ 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