MINGLING OF COLOURS. 



I I I 



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 beings to turn 

 round, the figures seen in the glass appear to execute the particular move- 

 ments which they represent in different positions. Let us designate by 

 means of the figures i, 2, 3, the different openings through which the eye 



Fig. 109. Top or experiments demonstrating Newton s theory of the mingling of colours. 



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 I, he will see first figure I, which appears in the glass to pass before 



his eyes ; then the rotation of the disc displaces opening I, 



and the cardboard intervenes, until opening 2 appears ; then 



figure 2 takes the place of figure I, 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 



Fig. no. Disc. 



