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 then lit up from behind, the objective intensity is evidently constant through the extent of each layer; nevertheless every strip appears darker at the edge touching a more transparent layer, and lighter at the edge in contact with a thicker layer. The dull tints of China ink, superposed in layers, will produce a similar effect. The phenomena are produced by means of rotative discs of most beautiful and delicate gradations of colour. Let us give the sectors of the disc the form represented by fig. 114, and make them black and white; and when in rotation we shall see a series of concentric rings of a shade that becomes darker and darker towards the centre. The angular surface of the dark portions is constant in each of these rings. The intensity, therefore, of each ring is uniform during rapid rotation; it is only between one ring and another that the intensity varies. Each ring also appears lighter on its inner side when it borders on a darker ring, and darker on its outer side when in contact with a lighter ring. If the differences of intensity in the rings are very slight, one can scarcely judge sometimes if the inner rings are darker than the outer; the eye is only struck by the periodical alternations of light and shade presented by the edges of the rings. If, instead of white and black, we take two different colours, each ring will present two colours on its two edges, although the colour of the rest of the ring will be uniform. Each of the constituent colours presents itself with more intensity on that edge of the ring which borders on another ring containing a smaller quantity of the colour. Thus, if we mix blue and yellow, and the blue predominates in the exterior and the yellow in the interior, every ring will appear yellow at its outer, and blue at its inner edge; and if the colours present together very slight differences, we may fall into the illusion which causes the differences really existing between the colours of the different rings to disappear, leaving instead, on a uniformly coloured background, the contrasting blue and yellow of the edges of the rings. It is very characteristic that in these cases we do not see the mixed colours, but seem to see the constituent colours separately, one beside the other, and one through the other.