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Report of the meeting of the physiological section at the annual meeting of the British Association, including a contribution by Charles Sherrington:

‘Professor Sherrington demonstrated the production of an intense colour of subjective origin by whirling a disc coloured with red on a black-white ground. The disc should contain a black sector of about 170°. From one edge of the black sector broad concentric vermilion arcs are carried into the peripheral white parts of the discs for about 90°; from the other edge of the disc similar arcs in the central part of the white field. On rotating the disc so that black follows red in the circumferential field, and precedes red in the central, the outer arcs appear a dull dark maroon, the inner a bright orange, and the spaces between the outer arcs appear deep blue-green, and the spaces between the inner arcs appear primrose-yellow, especially when viewed by a yellowish illumination. Prof. Sherrington offered an explanation for the phenomenon which he based on negative after-images heightened by simultaneous and successive contrast-effects which, as he demonstrated at the meeting, take effect even when the speed of translation of a surface prevents the mind from perceiving the space-relations of the distribution of the contrasted tints. It was pointed out that the disc is being used in this experiment similarly to a circular rheotome for summation of effects which taken singly would by reason of their small quantity would be imperceptible. It was also demonstrated that by whirling the disc at higher speed the phenomenon becomes altered, the reds and greys then matching over the whole surface of the disc, the greys assuming a pale green tint; when this is obtained it was urged that one factor of those previously acting, namely simultaneous contrast, had alone become valent.’ (pp. 19-20)