Page:The wonders of optics (1869).djvu/64

 with great rapidity by means of the finger and thumb and a couple of silk threads fixed at opposite sides of their circumference. On each of these discs a design is painted, one half appearing on one side, and the other half on the other, in such a manner that the two parts form a single picture. You may have, for instance, Harlequin on one side and Columbine on the other, but on turning the card you will see them together. The body of a Turk may be drawn on one side and his head on the other, and, by rotating the card, the head suddenly finds a pair of shoulders to fit it. A sentence may be divided in the same way, or the words, or even the letters, may be divided between the opposite sides of the card: in fact, like the phenakistiscope, the designs applicable to this little instrument are endless.

The third of these instruments, the phantascope, is constructed in accordance with the peculiar power possessed by the eyes of adapting themselves to the distance of the objects they are looking at. Everybody must have noticed that in order to see objects plainly that are placed at different distances we insensibly alter the position and focus of the eyes, and that, consequently, objects even in the same plane as those we are looking at are not perceived by us until something calls our attention to them, and causes us to alter the position and focus of our eyes and fix our gaze on them. For instance, in looking at a canary in a cage, we have but a confused idea of the wires, which we will suppose to be midway between the bird and the observer. But if anything attracts our attention to the wires we lose sight of the bird, or at any rate see it only as a confused mass. If this experiment is made with care, it will be perceived that the object seen confusedly is always double,—a fact that may be verified by interposing the finger between the eyes and any object. When we look at the finger, the distant object will seem