OTHER CAUSES OF COLOUR.
The colours of the spectrum are to the sense of sight
what the tones of the gamut are to the sense of hearing.
On the one hand, the differences in the lengths
of the sonorous waves constitute the variety of note perceptible
by the ear; on the other, the differences in the
lengths of the luminous waves constitute the variety of
colour perceptible by the eye. By and by, we shall
learn both the length and rapidity of these vibrations,
but it will be as well first to describe the experiments
made in this direction by the immortal Newton
himself.
Every one has, doubtless, at one period of his life, amused himself with blowing soap-bubbles by means of a tobacco-pipe and a little lather—a sufficiently childish amusement, you will possibly say, but one narrowly connected with the most intricate secrets of the science of optics. These little globes, so fragile that they disappear in a breath, hardly seem worthy of the attention of a thinker, and still less the examination of a philosopher; but it is nevertheless true that Newton made experiments on the colours shown on the surface of these apparently insignificant objects which ended in the most brilliant discoveries, just as on seeing an apple fall he began a train of thought which only terminated in the enunciation of the hypothesis of the earth's power of gravity.
