It was found that the dark-current pulses of photomultipliers are chiefly produced by the particles emerging from the glass envelope, i.e. if the envelope is at a more negative potential than the cathode, electrons are emitted by the envelope, and if the envelope is at a more positive potential than the cathode the electrons - produce scintillations in the glass envelope and a photon feedback takes place. By this mechanism the fact that the shield has a greater effect on the dark-current pulses of higher amplitudes can be satisfactorily explained.