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An electrooptical method for producing reverberation synthetically is described together with a summary of the development work and the various models which have been built. The basic principle consists in recording a fugitive sound pattern of the original program signal on the rim of a rotating phosphorcoated disk by means of a modulated light source and a simple optical system. The signal is picked up from the disk at later points through another simplified optical system and photocells. The logarithmical decay of the sound images on the phosphor as they pass the phototubes gives the required reverberation effect. This secondary signal is then mixed with the original program signal in any desired proportion. The method of modulating a high-pressure mercury-vapor lamp as an integral part of the aforesaid development work is described, and also the method of modulating a high-pressure mercury-vapor lamp with audio frequencies, together with a simplified optical system of high efficiency.