Indeed, distinct vision can be exercised in a very small fraction of a second. It was calculated by Professor Rood, and proved by experiment, that forty billionths of a second is sufficient time for the eye to distinguish letters on a printed page. It this instance the illuminating power was an electric spark from a Leyden Jar.
We have remarked upon the distinctness with which we can see an object when we direct our gaze upon it, and this appears a self-evident proposition; but have any of our readers remarked the curious fact that when they want to see a faint and particular star in the sky it will at once disappear when they gaze at it? The best way to see such very faint orbs as this is to look away from them,—a little to one side or the other,—and then the tiny point will become visible again to the eye. There is also a degree of phosphorescence in the eye, which any one who receives a blow upon that organ will readily admit. Even a simple pressure on the closed lid will show us a circle of light and “colours like a peacock’s tail,” as the great Newton expressed it. There are many occasions in which light is perceived in the eye—generally the result of muscular action; and the Irish term to “knock fire out of my eye” is founded upon philosophical facts.
We are many of us aware of “spots” on our eyes when our digestion is out of order, and the inability of the eye to see figures distinctly in a faint light—within a proper seeing distance, too—has often given rise to the “ghost.” These shadowy forms are nothing more than affections of the eye, and, as well remarked in Brewster’s Letters on Natural Magic, “are always white because no other colour can be seen.” The light is not sufficiently strong to enable the person to see distinctly; and as the eye passes from side to side, and strives to take in the figure, it naturally seems shadowy and indistinct, and appears to move as our eyes move. “When the eye dimly descries an inanimate object whose different parts reflect different degrees of light, its brighter parts may enable the spectator to keep up a continued view of it; but the disappearance and reappearance of its fainter parts, and the change of shape which ensues, will necessarily give it the semblance of a living form; and if it occupies a position which is unapproachable, and where animate objects cannot find their way, the mind will soon transfer to it a supernatural existence. In like manner a human figure shadowed forth in a feeble twilight may undergo similar changes, and after being distinctly seen while it is in a situation favourable for receiving and reflecting light, it may suddenly disappear in a position before and within the reach of the observer’s eye; and if this evanescence takes place in a path or roadway where there is no sideway by which the figure could escape, it is not easy for an ordinary mind to efface the impression which it cannot fail to receive.” This will account for many so-called “ghosts.”
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.
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.
But twelve years later, Professor Fraunhofer made a chart of these lines, which are still known by his name. Only, instead of the 576 he discovered, there are now thousands known to us! To Fraunhofer’s telescope Mr. Simms added a collimating lens, and so the Spectroscope was begun; and now we use a number of prisms and almost perfect instruments, dispersing the light through each. We have here an illustration of a simple Spectroscope, which is much used for chemical analysis (fig. 150).