LONDON. Royal Society, Feb. 10.—C. D. Ellis and W. A. Wooster: The photographic action of β-rays. The β-ray spectra of radio-active bodies were recorded as usual on a photographic plate, but homogeneous beams separated by magnetic deflexion were used. The reciprocity law is found to hold, and the form of the blackening curve is independent of the time of development and of the velocity of the particles between Hp 1450 and Hp 5830. The blackening curve follows a simple empirical formula which is identical with that holding for the blackening by X-rays, the constants in the formula showing the same variation with velocitv as holds for variation in frequency in the case of X-ravs.