Page:Spectropia, or, Surprising spectral illusions.djvu/10

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To see the spectres, it is only necessary to look steadily at the dot, or asterisk, which is to be found on each of the plates, for about a quarter of a minute, or while counting about twenty, the plate being well illuminated by either artificial or day light. Then turning the eyes to the ceiling, the wall, the sky, or better still to a white sheet hung on the wall of a darkened room (not totally dark), and looking rather steadily at any one point, the spectre will soon begin to make its appearance, increasing in intensity, and then gradually vanishing, to re-appear and again vanish; it will continue to-do so several times in succession, each reappearance being fainter than the one preceding. Winking the eyes, or passing a finger rapidly to and fro before them, will frequently hasten the appearance of the spectre, especially if the plate has been strongly illuminated.

Those who use gaslight will find it convenient, after having looked at the plate as above described, to extemporise a darkened room by having the gaslight turned low.

The size of the spectres will be determined by the distance of the observers from the plate, and from the surface against which they are seen; being larger the nearer the plate, and smaller the nearer the surface; so that short-sighted persons will see them larger than long-sighted, if both are equidistant, from the surface against which they are seen.

Should any one not be able to see the spectre's features, the reason will be, either that the eyes have been allowed to wander, or the head to move, while looking at the plate.

Many persons will see some one coloured spectre better than the others in consequence of their eyes not being equally sensitive to all colours.

The colours in the plate will be found to reverse themselves in the spectres, as explained elsewhere, the spectres always appearing of the complementary colour to that of the plate from which it is obtained. Thus, blue will appear orange, and orange blue, &c.