Page:Proceedings of the Royal Society of London Vol 69.djvu/134

126 whereas on the surrounding portions each green flash is followed by a white one, The area corresponding to the green disc appears purple, with a black spot in the middle.

Obviously this experiment falls under the first of the four classes of successive contrast described in my paper, i.e., the first stimulus excites a single colour-sensation, and the second excites all the sensations. The conditions are less simple than those of the second, third, or fourth classes described by me, and I have therefore repeated Dr. Shelford Bidwell's experiment, and have extended the same principle to the other classes of contrast referred to.

The following arrangement serves admirably for exhibiting the phenomenon. In front of the disc A is placed a piece of coloured glass (preferably green or blue) B, and behind it, at an angle of 45 to its plane, the silvered glass mirror C. Light passing through B is reflected from C parallel to the plane of the disc. In the path of the reflected rays is a second mirror, D, of unsilvered glass, so placed as to reflect rays from E in the same direction. E may conveniently be a black bead fixed on a wire or a black spot on a piece of glass.

The disc A, fig. 1, has two opposite sectors of about 30 cut out, and the positions of B and E are so regulated that on rotating the disc a flash of coloured light from B is seen first in the mirror, and then a

flash of white light of less intensity is reflected from D, and in the midst of it the image of the black bead, this being followed by a longer period of darkness. As long as the rate is not much more than six revolutions per second the light, when green glass is used, appears