Accidental colours, or ocular spectra, are, so to speak, illusions, and differently-coloured objects will, when our gaze is turned from them, give us different “spectra” or images. For instance, a violet object will, when we turn to a sheet of white paper, give us a yellow “spectrum”; orange will be blue; black and white will change respectively; red will become a blue-green. From a very strong white light the accidental colours will vary.
[Illustration: Fig. 149.—The Solar Spectrum.]
The Solar Spectrum is the name given to the coloured band formed by the decomposition of a beam of light into its elementary colours, of which there are seven. This is an easy experiment. A ray of light can be admitted into a darkened room through a hole in the shutter, and thus admitted will produce a white spot on the screen opposite, as at _g_ in the diagram (fig. 149). If we interpose a prism—a triangular piece of glass—the “drop” of a chandelier will do—we cause it to diverge from its direct line, and it will produce a longer streak of light lower down. This streak will exhibit the prismatic colours, or the “colours of the rainbow”; viz., red (at the top), orange, yellow, green, blue, indigo (blue), and violet last. These are the colours of the Solar Spectrum. The white light is thus decomposed, and it is called mixed light, because of the seven rays of which it is composed. These rays can be again collected and returned to the white light by means of a convex lens.
“White light,” said Sir Isaac Newton, “is composed of rays differently refrangible,” and as we can obtain the colours of the rainbow from white light, we can, by painting them on a circular plate and turning it rapidly round, make the plate appear white. Thus we can prove that the seven colours make “white” when intermingled. But Newton (1675) did not arrive at the great importance of his experiment. He made a round hole in the shutter, and found that the various colours overlapped each other. But, in 1802, Dr. Wollaston improved on this experiment, and by admitting the light through a tiny slit in the wood, procured an almost perfect spectrum of “simple” colours, each one perfectly distinct and divided by black lines.