A photoelectric spectrophotometer that is used to determine the ozone content of the atmosphere. The instrument compares the solar energy at two wavelengths in the absorption band of ozone by permitting the radiation of each to fall alternately upon a photocell. The stronger radiation is then attenuated by an optical wedge until the photoelectric system of the photometer indicates that equality of radiation exists. In this manner the ratio of the radiation intensity is obtained. The ozone content of the atmosphere is computed from this value. See Knudsen's tables.