Page:Proceedings of the Royal Society of London Vol 69.djvu/135

Rh purple, and the bead looks quite black against it. But when the disc is driven faster, at a certain speed both bead and glass look green, turning black and purple again respectively when the speed diminishes. If the intensity of the flash is reduced, there comes a point at which the phenomenon ceases to be observed, and if the area of the spot is increased, a limit is reached beyond which the effect no longer extends to its centre. My results agree as to these details completely with those obtained by Dr. Shelford Bidwell.

The essential difference between this experiment and those described in my paper consists in the use of the black spot, and the fact dis- covered by Dr. Shelford Bidwell, that the contrast effect may be so powerful in a spot not directly subjected to the first stimulus, is of the utmost importance, proving, as he points out, that the first sensation is not merely swamped by the second, but actually never comes into existence.

I do not, however, agree with him in thinking that no explanation of it can be afforded by the Young-Helmholtz theory, nor do I consider it warrants us in postulating an independent white sensation, and this view is supported by the following experiments, made on the lines laid down in my previous paper.

Two discs A and B, fig. 2, are carried by a shaft C about 100 cm, long. Each disc is composed of two equal circular sheets of metal, with a pair of sectors of 30 cut out at opposite ends of a diameter, so

FIG. 2.

that by rotating one over the other the aperture may be of any desired width, and fitted with clamping nuts so that the relative position of A and B on the shaft can be varied. D is an objective of 7 cm. focal length, and E is a plano-convex lens 10 cm. in diameter, which with the smaller lens F forms in effect a Campani eye-piece of about