By this time we must be thoroughly convinced that by throwing one coloured patch over another a compound colour can be formed; our next business is to demonstrate that the same effect can be produced by successive images of these same colours. Thus we can show that as a mixture of red and blue produces purple, when the two lights are superposed, so precisely the same purple can be produced by allowing the same two colours to strike the eye alternately, and in very rapid succession. We can make a match of the beautiful purple of permanganate of potash as before upon the screen, by placing one adjustable slit in the red and the other in the violet. If we place in front of the slits a disc cut out with equal angular apertures (Fig. 39), the slit S₁ will be covered when the slit S₂ is open, and _vice versâ_, and the two will never be uncovered at the same time when the card is turning round its centre. When this disc is caused to rotate rapidly, we shall have first a patch formed by the light coming through one slit, and then another formed by that coming through the other slit, thrown on the screen on the same place in rapid succession, and the effect on the eye should be precisely the same as if the disc was not there, save in the matter of intensity. Applying this artifice experimentally to the two slits which were used to give the colour of permanganate, the experiment tells us that such is the case. It would be going away from the intention of this work were the physiological aspect of this experiment dwelt upon; it need only be stated that an impression on the retina lasts an appreciable time, though short, and that the impression made by the blue patch has not had time to disappear before there is an impression made by the red patch, and so on. As the retina retains these two impressions together, they produce the impression of purple. Fig. 40.--Disc painted Blue and Red.