[Fig. 11] is a double pendulum arranged as shown. The pendulum A is heavily weighted, whilst the pendulum B is light, being only a string with a small weight attached. This difference in weight was made designedly, to prevent any great effect of the movement of B being shown on A, though that of A must necessarily exercise a great influence on B. The two pendulums are now of the same length. A is set in motion, and as it swings, B also begins to swing, and soon is oscillating with greater motion than A, and continues to do so. The length of the pendulum B is next shortened, and A is again set in motion. B takes up the motion, and increases its swing more and more, but now the two pendulums are in opposite phases, and the motion of A tends to diminish the swing of B, and continues to do so till, after an interval of time, B is once more at rest, when it again will start swinging. The fact is, that when A commences to swing, B also commences; and as long as B and A are moving in the same direction the impulses tend to make B increase its swing, but when they are moving in the opposite direction, or rather, perhaps it should be said, when A begins to start from the highest point of its swing downwards whilst B is travelling upwards, the swing of B will gradually diminish. This, of course, must happen when B is shorter or longer than A, since their times of oscillation are then different. We can now picture to ourselves that when in the perceiving apparatus in the retina the moving parts—probably molecules or atoms—arrive at a certain amplitude, there is then an impression of light, and that it is quite possible that not only those waves whose motion is exactly of the same period as that of the apparatus will set them in motion, but also those waves which are actually of a very different period. If such be the case, it can be seen that waves of light of some periods may set each of the three kinds of perceiving apparatus in motion, and that possibly the resulting impressions given by the sum of all three for a wave out of tune with any of them may be even greater than when the wave period is absolutely the same as one of them. For in the last case a maximum effect may be produced on one apparatus, and the effects on the other two may be insignificant; whilst in the first case the effects on two of them may be so large that their combined effects may have a larger value.
Fig. 12.
The following diagram ([Fig. 12]), made on the principle of Lissajou’s figures, shows graphically the motion of the pendulum. The pendulum, with a pen attached, was started by an independent pendulum, which had a different period, and the amplitude of the former registered itself on paper which moved by clockwork round the axis of suspension. As the two pendulums had different periods, the amplitude, as shown by the traces made, first increased and then diminished till there was no motion, and then started again. The trace is very instructive, and deserves attention. It will be noticed that the amplitude, or length of swing, increased rapidly at first, and then very gradually attained a maximum. Having attained this maximum, the amplitude diminished very slowly for some time, and finally came rather rapidly to zero, and the pendulum for an instant was at rest.
Fig. 13.
The top figure is the red sensation on the Young theory; the middle is the green sensation, and the lowest the violet or blue sensation.
With the notion in our minds that the perceiving apparatus might act in the way that the pendulum acts, we naturally apply it to the theories which early investigators on colour vision propounded. Thomas Young, whose name has already been mentioned, had propounded a theory of vision, which depended on the existence of only three colour sensations, and Von Helmholtz adopted it and explained the action of the three sensations in reference to the spectrum as shown in the diagram. These figures do not pretend to be absolute measures of the sensations, but only of the form which they might take ([Fig. 13]). The height of the curve at each part of the spectrum is supposed to represent the stimulation given to each apparatus by the different colours. Looking at the figures we see that each sensation has a place of maximum stimulation, and that the stimulation falls off more or less rapidly on each side of this maximum. It will, however, be noticed that whilst the green sensation takes very much the form of the pendulum amplitudes ([Fig. 12]) between its periods of rest, the other two differ from it. In the case of the red sensation, the stimulation falls very rapidly in the red as it reaches the limit of visibility of the spectrum, and in that of the blue sensation the steep descent is towards the extreme violet. When the three sensation theory is examined in the light of the careful measurements that have been made, the results tell us that these diagrams can only be taken as suggestive.