stituents, as we all know, by passing it through a prism. If we allow a ray of sun-light, or a beam of light obtained from an arc-lamp, to traverse a prism, after passing it through a lens and a narrow slit, and arrange the prism at a proper angle, we shall obtain a series of colours which, if received upon a white screen, spread themselves out into a long band, and range from a deep red to a very tender violet. This is the spectrum-band.
Below the red and beyond the violet are other rays, or other colours, which our eyes cannot see; but confining ourselves to those which are visible, and comparing this band of colours with the musical octave, we shall find at least one very remarkable point of resemblance.
If we get the physicist to measure the speed of the vibrations of the ether at the red and violet end of the band respectively, we shall find that, as in the case of the last note of the musical octave, the latter has approximately double the number of vibrations. The octave of colour is in fact practically complete, and a counterpart of the musical octave as regards
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