Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

with a single colour; and if two or more are superposed, with their slits placed one over the other, we obtain sectors, the size of which we may vary at will, so that we can modify in a continuous manner the proportions of the colours. The most perfect construction is that of Busold’s chromatic top (fig. 111), which should only be employed for very rapid rotations. The disc, which weighs 5 lbs., is made of an alloy of zinc and lead, about an inch and a quarter in diameter. The brass axis terminates at its lower end with a blunt point of untempered steel; the cylindrical part of the axis is roughened to encourage the adherence of the string; the axis is placed between the clamps of a vice, and a plate is put underneath; we then pull the string firmly with the right hand, and when the top is in motion it is separated from the clamps. By pulling the string very powerfully it is possible to obtain a speed of sixty turns in a second, and the movement will be kept up for three quarters of an hour.

      Fig. 108.—Spinning a top with coloured discs.

      Fig. 109.—Top for experiments demonstrating Newton’s theory of the mingling of colours.

      Fig. 110.—Disc.

      Besides tops, we may make use of different kinds of discs, with an axis rotating between two clamps; they are moved either by a kind of clock-work, or by the unrolling of a string, like the tops. Generally, however, these contrivances have this inconvenience, that the discs cannot be changed without stopping the instrument, and partly taking it to pieces. On the other hand, we have the advantage of being able to turn them on a vertical plane, so that we can conveniently carry on our experiments before a numerous auditory, which is a more difficult matter with tops. Montigny contrived to obtain the mingling of colours by means of a turning prism, which he caused to throw its shadow on a white screen. The Thaumatrope is a small rectangle of cardboard, which is made to rotate on an axis passing through the centres of the longest sides. We shall describe it at greater length when we come to consider a new apparatus known under the name of the Praxinoscope.

      Fig. 111.—Busold’s chromatic top.

      More complicated contrivances have also been constructed on the same principle, by which one may perceive the rotating disc through slits which turn at the same time. We will now describe the construction of some discs invented by Plateau under the name of the Phenakistoscope. These discs are made of strong cardboard, from six to ten inches in diameter (fig. 112), on which a certain number of figures (eight to twelve) are placed in circles at an equal distance from each other, presenting the successive phases of a periodical movement. This disc is placed on another opaque circle of rather larger diameter, which has on its margin as many openings as the first disc has figures. The two discs are placed one on the other, and are fixed in the centre by means of a screw at the anterior extremity of a small iron axis, the other end being fitted into a handle. To make use of this contrivance we place ourselves in front of the glass, towards which we turn the disc with the figures, placing the eye so as to see the figures through one of the holes of the large disc. Directly the apparatus begins to turn round, the figures seen in the glass appear to execute the particular movements which they represent in different positions. Let us designate by means of the figures 1, 2, 3, the different openings through which the eye successively looks, and point out by the same numbers the figures in the radiuses thus numbered. If the experimenter looks in the glass through opening 1, he will see first figure 1, which appears in the glass to pass before his eyes; then the rotation of the disc displaces opening 1, and the cardboard intervenes, until opening 2 appears; then figure 2 takes the place of figure 1, until it in turn disappears, and opening 3 presents figure 3 to view. If these figures were all similar, the spectator would have but a series of visual impressions, separate but alike, which by a sufficiently rapid rotation mingle together in one durable impression like a perfectly immovable object. If, on the contrary, the figures differ slightly from each other, the luminous sensations will also mingle in a single object, which will however appear to be modified in a continuous manner, conformably with the differences of successive images. With a difference of speed, we obtain a new series of phenomena. A most simple contrivance of this kind is a top of C. B. Dancer, of Manchester (fig. 113). It will be seen that the axis carries another disc, pierced with openings of different shapes, to the edge of which a thread is attached. This second disc is carried along by the friction of the axis, but its rotation is less rapid because of the great resistance offered by the air to the piece of thread which participates in the movement. If the lower disc has several differently-coloured sectors, they produce a very motley appearance, which seems to move sometimes by leaps, and sometimes by continuous motion. We must distinguish between the phenomena of successive contrast and simultaneous contrast.

      Fig. 112.—Rotating disc.

      Fig. 113.—Mr. Dancer’s top.

      Phenomena of successive contrast develop what are called accidental images. If we fix our eyes for a considerable time on a coloured object, and then suddenly direct them towards a uniform white surface, we experience the sensation of the object as it is, but it appears coloured with a complementary tint; that is to say, it has the colour which, superposed on the genuine tint, we obtain from pure white. Thus a red object produces a consecutive green object. The experiment can be tried by gazing at the sun when it is setting, and then directing one’s eyes towards a white wall in the same direction.

      Phenomena of simultaneous contrast arise from the influence exercised over each other by different shades and colours which we see simultaneously. That we may be certain that we have really obtained phenomena of this kind, the experiments must be arranged in such a manner that accidental images are not produced, and that the part of the retina affected by the sensation of colour does not receive, even momentarily, a passing image.

      Fig. 114.—Disc, which exhibits, when in rotation, a series of concentric rings.

      The phenomena of simultaneous contrast appear with the greatest clearness with slight differences of colour, and are therefore exactly the contrary of phenomena of successive contrast, which are favoured by strong oppositions of colour and light. We can, in general, characterise phenomena of simultaneous contrast as governed by this law, common to all perceptions of the senses: the differences clearly perceived appear greater than the differences equal to them, but perceived with greater difficulty, either because they only affect the observation in an uncertain manner, or that the memory fails to judge of them. A man of middle height appears small beside a tall man, because at the moment it is forcibly impressed on us that there are taller men than he, and we lose sight of the fact that there are smaller. The same man of medium height appears tall beside a man of small stature. We can easily make experiments on simultaneous contrast with a sheet of transparent paper. We fasten together a sheet of green and a sheet of rose-coloured paper, so as to obtain a sheet half red and half green. On the line of separation between the two colours we place a strip of grey paper, and cover the whole with a sheet of thin letter-paper of the same size. The grey strip will then appear red at the edge touching the green, and green at the edge touching the red; the centre presenting an intermediate shade. It presents a still more decided appearance if the grey strip is perpendicular with the line of separation of the two colours; the piece of grey then stretching into the green will present as deep a red as the red foundation on the other side. If the line of grey colour exactly covers the line of separation between the two colours, the contrasting colour is more feeble; the edges of the grey paper then present complementary strips of colour. Similar effects may be obtained by superposing, in gradually diminishing layers, strips of thin paper, so as to form successive bands of different thicknesses. If it is