The development of a new photometric principle has led to the design of an integrating photometer which appears eminently adapted to the measurement of the total intensity in X-ray crystal reflections, and which is capable of extension to other types of radiation also. It is possible to obtain direct proportionality between X-ray intensity and α-ray ionization, when a stripped carbon tissue of the original negative is interposed between a source of α-particles and an ionization chamber. After a short account of its working principles, a description is given of such an α-ray photometer which has been designed and used at the Davy Faraday Laboratory of the Royal Institution.