Cathode-ray oscillograms of stellar and planetary scintillation have been recorded over a period of a year. For this investigation selected multiplier photocells (type RCA 931–A) of special sensitivity and high signal/noise ratio have been used at the Cassegrain focus of the 36-inch reflector of the Royal Observatory. The output voltages, after amplification, are applied to the beam of an oscillograph, the “Y -deflections” of the spot being recorded on moving film. The sensitivity and dark current of the cells and the time constant of the circuit are discussed. Scintillation in integrated light is exhibited as a function of altitude and of seeing conditions. Scintillation amplitudes of 5 to 10 per cent are always present in the zenith, when recording with the full aperture of 36 inches. The amplitude of scintillation is found to vary with telescope aperture A approximately in the ratio 1/A, but appears to reach a maximum at an aperture near 3 inches. Under normal conditions at this aperture, high-altitude stars show sudden flashes of light, lasting about one-hundredth of a second, which cause momentary increases in brightness of 1 to 2 magnitudes. The average size (3 inches) and distance (5 km) of the atmospheric irregularities causing scintillation have been deduced from a comparison of the scintillation patterns of stars, planets and satellites. Some experiments on colour scintillation, using “monochromatic” light, are described. An attempt has also been made to imitate the main features of scintillation by generating refractional irregularities in a model atmosphere of water.