It is a curious circumstance, that when the image of an object is impressed upon the retina only for a few moments, the picture which is left is exactly of the same colour with the object. If we look, for example, at a window at some distance from the eye, and then transfer the eye quickly to the wall, we shall see it distinctly, but momentarily, with _light_ panes and _dark_ bars; but in a space of time incalculably short, this picture is succeeded by the spectral impression of the window, which will consist of _black_ panes and _white_ bars. The similar spectrum, or that of the same colour as the object, is finely seen in the experiment of forming luminous circles by whirling round a burning stick, in which case the circles are always red. In virtue of this property of the eye, an object may be seen in many places at once; and we may even exhibit at the same instant the two opposite sides of the same object, or two pictures painted on the opposite sides of a piece of card. It was found by a French philosopher, M. D’Arcet, that the impression of light continued on the retina about the eighth part of a second after the luminous body was withdrawn, and upon this principle Dr. Paris has constructed the pretty little instrument called the _Thaumatrope_, or the _Wonder-turner_. It consists of a number of circular pieces of card, about two or three inches broad, which may be twirled round with great velocity by the application of the fore-finger and thumb of each hand to pieces of silk string attached to opposite points of their circumference. On each side of the circular piece of card is painted part of a picture, or a part of a figure, in such a manner that the two parts would form a group or a whole figure, if we could see both sides at once. Harlequin, for example, is painted on one side, and Columbine on the other, so that by twirling round the card the two are seen at the same time in their usual mode of combination. The body of a Turk is drawn on one side, and his head on the reverse, and by the rotation of the card the head is replaced upon his shoulders. The principle of this illusion may be extended to many other contrivances. Part of a sentence may be written on one side of a card, and the rest on the reverse. Particular letters may be given on one side, and others upon the other, or even halves or parts of each letter may be put upon each side, or all these contrivances may be combined, so that the sentiment which they express can be understood only when all the scattered parts are united by the revolution of the card.