344 W. MCDOUGALL : at an acute angle opening towards a point on the observer's right hand, and at the same moment A and B seem to begin to move away and towards the left while C and D begin to move towards a point on the right of the observer. One of these phases is of course illusory, the other corresponds to objective reality. 1 During the former phase, A seems nearer to the observer than B and D seems nearer than C, but during the second phase B seems nearer than A and C seerna nearer than D, and so long as the observer stands at a distance of thirty yards or more the disparation of the images of the sails on the two retinae is insufficient to render possible binocular perception of the relative distances of the sails. But it is clear that, if the sails are made small and are viewed from a point so near that the conditions of binocular percep- tion of distance are satisfied, the observer will perceive the B FIG. 11. true relative distances of A and B and of C and D and that this must strongly favour the predominance of that mode of perception which corresponds to the reality. I have therefore constructed a small rough copy of a windmill. It consists of four brass rods, each 20 cm. in length, fixed at right angles to one another on one end of a long steel rod and in a plane vertical to this rod. The steel rod or axis is held horizontally in strong bearings which leave it free to rotate, and on the end remote from the four-rayed star formed by the brass roda a pulley-surface is fixed. A cord from a motor passing over this surface keeps the brass rods in rotation in a vertical plane at any desired speed. This wheel or imitation windmill is- 1 Since windmills may still be seen outside laboratories I venture to suggest that some anti-realistic philosopher should study these curious appearances and should tell us whether he recognises any difference between the degrees of reality or truth of the two appearances, and if so,, how he proposes to account for it.
