(1) It is generally assumed that the vacuum photo-cell has no appreciable time lag. The electrons start out from the surface as soon as the light falls on it. The most important investigation on the subject is that of Marx and Lichtenecker (1913). By means of a mirror rotated 170 times per second an image of a narrow slit was swept over the cathode of a photo-cell at a distance of 8·3 metres from the mirror; the cathode was a potassium one sensitised with hydrogen. The electrometer reading was noted, and then the mirror was rotated at a much slower speed and the electrometer read again. No appreciable difference was recorded. In the words of the authors an alteration of the time of illumination from 4·38 × 10-3 to 1·46 × 10-7 sec., the intensity of radiation being 0·56 erg/sec. cm.2, had no effect on the result to an accuracy of one or two per cent. Theoretical deductions have been drawn from this statement which have been much quoted. It seemed to me that the statement itself was worth verifying, and this paper is the record of an attempt to verify it and push the limit of accuracy further.