Measurements are reported on the color sensitivities of various photoelectric tubes having cathodes made of alkali metals. A composite surface of R‐Ag‐RO‐Ag of any alkali metal R shows a higher sensitivity over a wider range of wave‐lengths than R‐RO‐Ag and has two peaks, one between 330 and 370 m&mgr;, and one at about 500 m&mgr; for potassium and 550 m&mgr; for rubidium and between 700 and 800 m&mgr; for caesium. If a gas filled lamp at 2700°K is used as a light source, photoelectric currents from vacuum photo‐tubes are obtained sometimes as large as 29&mgr;a per lumen for potassium, and 10–15&mgr;a per lumen for rubidium, and 40–50&mgr;a per lumen for caesium. It is also reported that the potassium cell in combination with a proper filter would constitute a means for an ideal photometer.