Discontinuities have been observed in the saturation curves of most of a number of vacuum photoemissive cells of the special photometric types. In these cases, in certain circumstances, the cells can operate in either of two stable states for which the observed photo-currents, for the same light intensity, can differ by a significant amount. A reason is suggested, in terms of variations in potential of the inner bulb surface arising from secondary emission processes at the bulb and leakage of this surface to the anode. This is confirmed by a series of experiments. The main conclusion is that the best conditions, for saturation and freedom from irregular behaviour, are ensured if the bulb surface has high conductivity, and is in good connexion with the anode but well insulated from the cathode. Paradoxically, good saturation cannot be expected over the normal range of voltage if the bulb surface is highly insulating, partly because the bulb then remains at cathode potential and considerably depresses the field strength at the cathode, and partly because - at that potential - it can reflect an appreciable fraction of the photoelectrons back into the cathode.