In order to complete the history of the illusions which originate in the eye, it will be necessary to give some account of the phenomena called _ocular spectra_, or _accidental colours_. If we cut a figure out of red paper, and, placing it on a sheet of white paper, view it steadily for some seconds with one or both eyes fixed on a particular part of it, we shall observe the red colour to become less brilliant. If we then turn the eye from the red figure upon the white paper, we shall see a distinct _green_ figure, which is the _spectrum_, or accidental colour of the _red_ figure. With differently coloured figures we shall observe differently coloured spectra, as in the following table:-- COLOUR OF THE COLOUR OF THE ORIGINAL FIGURES. SPECTRAL FIGURES. Red, Bluish-green. Orange, Blue. Yellow, Indigo. Green, Reddish-violet. Blue, Orange-red. Indigo, Orange-yellow. Violet, Yellow. White, Black. Black, White. The two last of these experiments, viz., white and black figures, may be satisfactorily made by using a white medallion on a dark ground, and a black profile figure. The spectrum of the former will be found to be black, and that of the latter white. These ocular spectra often show themselves without any effort on our part, and even without our knowledge. In a highly painted room, illuminated by the sun, those parts of the furniture on which the sun does not directly fall have always the opposite or accidental colour. If the sun shines through a chink in a _red_ window-curtain, its light will appear _green_, varying as in the above table, with the colour of the curtain; and if we look at the image of a candle, reflected from the water in a _blue_ finger-glass, it will appear _yellow_. Whenever, in short, the eye is affected with one prevailing colour, it sees at the same time the spectral or accidental colour, just as when a musical string is vibrating, the ear hears at the same time its fundamental and its harmonic sounds. If the prevailing light is _white_ and _very strong_, the spectra which it produces are no longer black, but of various colours in succession. If we look at the sun, for example, when near the horizon, or when reflected from glass or water so as to moderate its brilliancy, and keep the eye upon it steadily for a few seconds, we shall see, even for hours afterwards, and whether the eyes are open or shut, a spectrum of the sun varying in its colours. At first, with the eye open, it is _brownish-red_ with a _sky-blue_ border, and when the eye is shut, it is _green_ with a _red_ border. The _red_ becomes more brilliant, and the _blue_ more vivid, till the impression is gradually worn off; but even when they become very faint, they may be revived by a gentle pressure on the eyeball.