Since the beginning of 1941 measurements of the amount of ozone in the atmosphere have been made almost daily at New York University. The instrument used is a photoelectric spectrophotometer designed and calibrated by Dr. G. M. B. Dobson [see 1 of ”References“ at end of paper[ in England, and obtained by means of a generous grant made by the Research Corporation of New York City for the purpose. The amount of ozone in the atmosphere is determined by comparing absorption by the atmosphere of radiation from the Sun of wave-lengths 3110 Å and 3300 Å. The shorter wave-length is more strongly absorbed by ozone and thus from measurements of the relative intensities of these wave-lengths received at the Earth's surface the amount of ozone in unit vertical column of the atmosphere may be calculated. This is usually expressed in centimeters of ozone gas at normal temperature and pressure. In the instrument provision has also been made for measurement of the intensity of a third wave-length of 4450 Å which, in connection with the other wave-lengths, permits measurements using light from the cloudy zenith-sky, an attribute which is quite valuable during periods of cloudy weather.