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. [Illustration: Fig. 149.—The Solar Spectrum.]