Fig. 39.—Disc to cause alternate opening and closing of two Slits.
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.
For experiments in colour this duration of impressions is of great value, for we can take advantage of it to compound the colours of pigments together in a very simple manner. For instance, we can take a circular disc painted in sectors with blue and red (Fig. 40), and produce a purple by causing it to rotate round its centre. Small discs of two inches in diameter may be painted with different coloured sectors, and if a pin be passed through the centre, a smart movement of a finger at the periphery will cause it to rotate sufficiently quickly to make the colours blend. A more convenient plan for exact work is, however, to have an electro-motor similar to that which moves the rotating movable sectors ([Fig. 41]), and at the end of the spindle to fix a cap with a screw and nut attached. The disc, perforated at the centre with a clean-cut hole, can be slipt over the screw, and fastened by the circular nut. When the armature rotates, the disc also rotates at the same speed, and the colours thus blend without any exertion on the part of the observer. Ordinary tops can also be used, but it is somewhat fatiguing to have to wind them up and start them afresh for each experiment. The motor shown in the figure rotates sufficiently rapidly, with discs of eight inches in diameter, to blend colours. It may here be remarked that the stronger the light in which such sectors rotate, the quicker the rotation should be. Too slow a rotation allows a scintillation which is destructive of accuracy of reading. To blend some colours together also requires more rapid rotation than with others. The brighter the colour the more rapid it should be. We learn from this that the diminution of the more intense impressions on the retina is more rapid at first than of the feebler.
Fig. 41.—Electro-motor with Discs attached.
