Quite intense colours can be produced by pulsing coloured or white light. Flashing coloured lights can produce other colours: for example, flashing monochromatic sodium yellow light may produce any spectral hue (and brown, which is not in the spectrum), though not usually with very high saturation. These phenomena are most easily seen with a spinning disc, or top, having black and white sector rings (as in Fig. 1). This gives intermittent stimulation to retinal regions as it is rotated, quite slowly, to give a flicker rate below the fusion frequency. The colours seen depend on the widths and arrangement of the black sectors and the rate of rotation. There are considerable individual differences for what is seen under given conditions.
Fig. 1
Although generally called 'Benham's top', after C. E. Benham (1894), the basic effect goes back to a French monk, Benedict Prévost, who, in 1826, observed colours — like a heavenly light on his fingers — when he waved his hands about in the cloisters. Finding that this also happened with white cardboard, he realized that it has a physiological origin, in the eye, and attributed it to different rates of action of specific colour mechanisms of the retina. He was essentially correct. It is remarkable that Prévost's discovery was forgotten, and the effect was rediscovered no less than twelve times: by
Gustav Fechner in 1838 and then by others. John Smith, in 1859, thought that the effects were 'objective' by changing the light itself, and so he (incorrectly) challenged
Newton's account of light and colour. The third rediscovery was made by Sir
David Brewster in 1861. (The history is given fully in Cohen and Gordon 1949.)
Hermann von Helmholtz carried out systematic observations, noting that a white rotating sector is red on the leading edge and blue on the trailing edge, and in dim light the red becomes yellow and the blue violet. In very bright light the red becomes pinker and the blue greenish. Further observations were made by
S. Bidwell, who discovered a related effect, Bidwell's ghost.
These subjective colours have been shown successfully on black and white television, but they are a little too weak for commercial uses. They are due to different time constants of the colour receptor systems of the eye, but they are rather too variable for precise measurements. The effects are interesting but not particularly useful.
(Published 1987)— Richard L. Gregory
Bibliography- Benham, C. E. (1894). 'The artificial spectrum top'. Nature, 51.
- Bidwell, S. (1896). 'On subjective colour phenomena attending sudden changes in illumination'. Proceedings of the Royal Society, 60.
- Cohen, J., and Gordon, D. A. (1949). 'The Prévost–Fechner–Benham subjective colours'. Psychological Bulletin, 46.
