Design for the phenakistiscope. The spectator is supposed to be looking towards a mirror through the slits. It is supported by a handle through the centre, round which it is twirled by the other hand.
In the "Journal of the Royal Institution" Mr. Faraday has described some very interesting experiments and optical illusions produced by the revolution of wheels in different directions and velocities. The wheels are made of cardboard, and by cutting out two cog wheels of an equal size, and placing one above the other on a pin, the usual hazy tint when the cogs are acting is apparent when they are whirled round; but if the two cog wheels are made to move in opposite directions, there will be the extraordinary appearance of a fixed spectral wheel. If the cogs are cut in a slanting direction on both wheels, the spectral wheel will exhibit slanting cogs; but if one wheel is turned so that the cogs shall point in opposite directions, then the spectral wheel will have straight cogs. A number of such wheels set in motion in a darkened room, and illuminated suddenly with the light from the electric spark, appear to stand perfectly still, although moving with a great velocity. An expensive instrument has been constructed by Duboscq, for the purpose of showing the usual phenakistiscope effects on the screen with the magic lantern; a very limited picture, however, is shown, and there is still great room for the improvement of the apparatus. (Fig. 305.)
Fig. 305.
Phenakistiscope made by Duboscq, of Paris. No. 1. Apparatus in elevation with the condensers. No. 2. Section of the apparatus. a. The light. b. Condenser, or plano-convex lens. c. Round glass disc with design painted on it. d. Wooden disc with four double-convex lenses placed at equal distances from each other, so as to coincide with c, whilst rotating. Both the latter and c rotate, and the picture is focussed on the disc by the lenses f. No. 3. Glass plate, with device painted thereon.
IX. The Thaumatrope.
This very simple toy was invented by the late Dr. Paris, who gave it an appropriate name, compounded of the Greek words, θαυμα, wonder, τρεπω, to turn. The duration of the impressions of light on the eye is very apparent whilst using this toy, which is usually made of a circular piece of cardboard, having on one side a painting of a man's head, and on the other a hat; or a picture of a lighted candle on one face of the cardboard, and an extinguisher on the other; or a gate, and a horseman leaping it. Each pair of designs painted on opposite sides of the cardboard appear to be one when twisted round by strings tied to the opposite edges of the cardboard circle. The rationale of this experiment being, that the picture of one design—such as the head and face—is retained by the eye until the hat appears, and being mutually impressed upon the nerve of vision at very nearly the same instant of time, they appear as one picture.
X. The Kalotrope.
This is an optical arrangement by Mr. Thomas Rose, of Glasgow, primarily designed for showing the illusions of the phenakistiscope and kindred devices to a numerous audience; but more remarkable for its presentations of very beautiful spectra, composed of the multiplication, combination, and involution of simple figures disposed around a disc. The arrangement consists of a movement for giving considerable velocity to two concentric wheels, working nearly in contact, and moving in contrary directions. But the only part of the apparatus that requires special explanation and illustration is the device disc and the disc of apertures; the first of which is placed on the hinder wheel, and the second on the front wheel. We give figures of the two discs, premising, however, that each is capable of an almost infinite variety of characters. No. 1 (Fig. 306) presents in its four quadrants the perforations for four distinct discs of apertures; and No. 2 is a device disc, consisting of twelve equidistant black balls. Under a the balls will be presented as twenty-four ovals; under b, as forty-eight involved figures, beautifully variegated; under c, as an elaborate lacework; and under d, as a rich variegation of form and colour. Every fresh disc of devices and disc of apertures of course opens up a new field of effect. Thus, if we take a disc bearing twelve repeats of a ball in the interior of a ring, each repeat being so painted that its position is advanced in the ring until it reaches in the twelfth ring the point whence it started, and place this on the back disc of the kalotrope, having previously removed the first one, no effect is observed when the wheel is rotated beyond the spreading out of the design and general appearance of hazy black circles. When, however, the disc, with twelve slits or apertures, is now placed on the front wheel, and the two rotated in opposite directions, then the whole figure starts as it were into existence, and each ball apparently moves round the interior of its circle. The apparatus was produced at the Royal Polytechnic Institution by the author, and excited much interest. (Fig. 306.)