Infrared photometry has been practiced for many years by photoelectric methods ; however, the necessity in the past for employing diode photocells has restricted photometry in the infrared to those observers versed in the highly specialized electronic techniques required. The picture has changed within the last few years by the gradual appearance of good photomultipliers with the infrared-sensitive CsO-Ag cathode. Good infrared multipliers were first made in the laboratory of Prof. A. Lallemand

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... at the Paris Observatory, who has made these multipliers available to astronomers throughout the world. More recently, perfected infrared-sensitive multipliers have been placed on the market in the United States by the Farnsworth Electronics Company (their 16 M 1 ), the DuMont Laboratories, Inc. (their K 1613), and most recently, by the Radio Corporation of America (their 7102). All of these multipliers are of end-on configuration with semi-transparent cathodes. Several infrared multipliers made by Prof. Lallemand have been in use at the Lick Observatory for three years for multicolor photometry, and our experience was recently extended by thoroughly testing a sample of the DuMont K 1613. Inasmuch as no performance data on infrared-sensitive multipliers are available, it seems timely to present the results of some of our experience. Infrared-sensitive multipliers are practically useless unless they are refrigerated, because of the large thermal emission from the cathode. This thermal emission can be so large that it alone can load the anode with its full allowable output current, leaving nothing for the light to be measured. Furthermore, the shot noise from the large dark current is so great that unrefrigerated multipliers are not useful for measuring light in the brightness range commonly found in astronomical photometry. The tests reported upon here were made on multipliers encased in well-built, her-* Contributions from the Lick Observatory, Ser. II, No. 89.