234,808. Toulon, P. M. G., and Soc. de Recherches et de Perfectionnements. Industriels. May 27, 1924, [Convention date]. Addition to 209,426, [Class 30 (i), Electric lamps, Arc &c.]. Optical apparatus. - The parent Specification and Specification 209,096, [Class 39 (i), Electric lamps, Arc &c.], describe a mercury vapour lamp in which ionization is kept up by a continuouslyapplied potential difference between mercury electrodes 5, 6 and the current between the electrodes 5, 3 is controlled by varying the potential of an exterior electrode 12.. According to the present invention, the variations in the luminosity of the mercury lamp may be employed, inter alia, for transmitting speech in a photophone system, a photographic sound record being produced at the receiving station. In the arrangement shown in Fig. 3, the intensity of the light beam 40 fluctuates in accordance with speech currents through the microphone 24, a screen 39 being employed if desired to cut out the visible spectrum. At the receiving station the light-fluctuations are translated into sound by a photo-electric cell, or are photographically recorded. Alternating current of fairly high frequency is supplied at 31 so that through the transformer secondary 32<1> an alternating voltage is applied between the electrodes 3, 5. Another secondary 32<11> is arranged in the plate circuit of a valve 27 together with a choke coil 36<1> which causes the plate current to lag by nearly 90‹ behind the voltage applied to the electrode 3. A further secondary 32<111> supplies current to a resistance 37, this current being substantially in phase with the applied E.M.F. The resistances 37, 30 are connected in series between the cathode 5 and control electrode 12, so that the voltage applied to the latter is the vector sum of the voltage drops in the resistances 37, 30. Hence the voltage of the electrode 12 differs in phase from that of the electrode 3 by an amount depending on the relative strengths of the currents in the resistances 37, 30 since these differ from one another in phase by about 90‹. Thus the microphone 24, which controls the strength of the component in 30. thereby controls the phase in 12, and so varies the luminosity of the lamp. The variations in luminosity may be recorded photographically so as to produce a sound record.