In his recent paper on the Photometry of Lights of Different Colours Mr H. Ives remarks:—“No satisfactory theory of the action of the flicker photometer can be said to exist. What does it actually measure? We may assume the existence of a ‘luminosity sense’ distinct from the colour sense.… If, for instance, there exists a physiological process called into action both by coloured and uncoloured light, a measure of this would be a measure of a common property.”

Very many years ago it occurred to me that the adjustment of the iris afforded just such a “physiological process”. The iris contracts when the eye is exposed to a bright red or to a bright green light. There must therefore be some relative brightness of the two lights which tends equally to close the iris, and this may afford the measure required. The flicker adjustment is complete when the iris has no tendency to alter under the alternating illumination.

This question was brought home to me very forcibly, when in 1875 I fitted the whole area of the window of a small room with revolving sectors after the manner of Talbot. The intention was to observe, more conveniently than when the eye is at a small hole, the movements of vibrating bodies. The apparatus served this purpose well enough; but incidentally I was much struck with the remarkably disagreeable and even painful sensations experienced when at the beginning or end of operations the slits were revolving slowly so as to generate flashes at the rate of perhaps 3 or 4 per second.