It is easy to place designs on the disc, even when in motion, or to make any desired modification, by superposing on the first disc another disc with sectors, of which we can vary the position by slightly touching it, or even blowing on it, thus producing during the rotation of the disc very varied modifications. If, for instance, we place on a disc covered with blue and red sectors of equal size, a black disc, of which the sectors are alternately filled in or empty, the disc, as it turns round, will appear blue if the black sectors of the upper disc exactly cover the [red] sectors of the lower disc; and it appears red, if, on the contrary, the blue sectors are covered with the black; while in the intermediate positions we obtain different mixtures of red and white, and during the rotation of the disc may vary the colour insensibly by a gentle touch. By dividing the different sectors with broken or curved lines, instead of straight ones, we can produce an arrangement of coloured rings of great variety and beauty. To give the top greater speed, we set it in motion by drawing a string twined round its stem. The simplest method, as shown in fig. 108, consists in the employment of a handle similar to that of the German top. It is a hollow cylinder of wood set into a handle with two circular holes; and at right angles with these is a groove for the passage of the string. The stem of the top is passed through the holes of the cylinder, one end of the string is fixed in the small hole in the stem, and is rolled round by turning the top in the hand. The part of the stem on which the string is twisted becomes sufficiently thick for the top to remain suspended to the handle; then holding it a little above the table, and giving the string a powerful pull, we set the top in motion, and as the string unrolls it falls on the table, where it will continue its rotation for some time. The top represented in fig. 109 is constructed so that the discs may be firmly pressed by the stem, which is necessary in experiments for demonstrating Newton’s theory of the mingling of colours. We make use for this purpose of a variety of discs, made of strong paper of different sizes, having an opening in the centre and a slit, as in fig. 110; each of the discs is covered uniformly 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.