122 COLOURED PHOTOMETRY [36i It is clear, I think, that we have here a common element in various! coloured lights, such as might serve as the basis of coloured photometry I suppose that there would be no particular difficulty in observing th movements of an iris, and I would suggest that experiments be undertake: to ascertain whether in fact the flicker match coincides with quiescenc of the iris. Should this prove to be the case, the view suggested would b amply confirmed; otherwise, it would be necessary to turn to some of th * other possibilities discussed by Mr Ives. [1913. Mr H. C. Stevens (Phil. Mag. Vol. xxvi. p. 180, 1912), in con nexion with the above suggestion, describes an experiment in which th musculus sphincter pupillae was paralysed with atropine, without changinj " in any observable particular " the appearance of flicker. This observatio] may prove that an actual movement of the iris is not necessary to th sensation of flicker, but .it can hardly be said that the iris has no tendency to alter because it is prevented from doing so by the paralysis of thi muscle. There must be more than one step between the impression upoi the retina which initiates a message to close the iris and the actual closing thereof. The flicker adjustment may, so far as appears, correspond to tin absence of such messages.]reeable 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. I soon learned in self-defence to keep my eyes closed during this phase; and I attributed the discomfort to a vain attempt on the part of the iris to adjust itself to fluctuating conditions.