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A type of phototube is described in which the secondary electron emission from an auxiliary cathode (bombarded by the photo-electrons) is utilized to obtain amplification of the primary photocurrent. Phenomena of secondary emission, particularly as applied to the vacuum phototube, are discussed. The operating performance of a typical developmental embodiment is illustrated, and it is shown that its static sensitivity is comparable with that of a corresponding gas phototube; that as regards fidelity, it retains the freedom of the vacuum phototube from the considerable loss in response at the higher audio frequencies which is inherent in the gas phototube; and that on the basis of noise produced by the microscopic fluctuations of its current, it is somewhat superior to the comparable gas tube, and approximately equivalent to a vacuum phototube having the same emission and followed by an amplifier having an over-all gain equivalent to the secondary emission amplification. Life tests indicate that the stability of these tubes is entirely comparable with that of the vacuum phototube, both as regards secondary emitter and photocathode behavior. In addition to the usual applications, various incidental uses of these phototubes are suggested.