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. [Illustration: 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. CHAPTER IV.