For a series of four screens of lead, each of which was 1·5 m.m. thick, the ratio of the radiation transmitted to the radiation received was given for the successive screens by the following numbers:— 0·09 0·78 0·84 0·82 The results of these experiments show that when the thickness of the lead traversed increases from 0·1 m.m. to 6 m.m., the penetrating power of the radiation increases. I found that, under the experimental conditions mentioned, a screen of lead 1·8 c.m. thick transmits 2 per cent of the radiation it receives; a screen of lead 5·3 c.m. thick transmits 0·4 per cent of the radiation it receives. I also found that the radiation transmitted by a thickness of lead of 1·5 m.m. consists largely of rays capable of deflection (cathode order). The latter are therefore capable of traversing not only great distances in the air, but also considerable thicknesses of very absorbent solids, such as lead. In investigating with the apparatus of Fig. 2 the absorption exercised by an aluminium screen 0·01 m.m. thick upon the total radiation of radium, the screen being always placed at the same distance from the radiating body, and the condenser being placed at a variable distance, A D, the results obtained are the sum of those due to the three groups of the radiation. At a long distance the penetrating rays predominate, and the absorption is slight; at a short distance the α-rays predominate, and the absorption becomes less with nearer approach to the substance; for an intermediate distance the absorption passes through a maximum and the penetration through a minimum. Distance A D 7·1 6·5 6·0 5·1 3·4 Percentage of rays transmitted by aluminium 91 82 58 41 48 Certain experiments made in connection with absorption always demonstrate a certain similarity between the α-rays and the β-rays. Thus it was that M. Becquerel discovered that the absorbent action of a solid screen upon the β-rays increases with the distance of the screen from the source, such that if the rays are subjected to a magnetic field, as in Fig. 4, a screen placed in contact with the source of radiation allows a larger portion of the magnetic spectrum to be in evidence than does the same screen placed upon the photographic plate. This variation of the absorbent effect of the screen with the distance of the screen from the source is similar to that which occurs with the α-rays; this has been verified by MM. Meyer and von Schweidler, who operated by means of the fluoroscopic method; M. Curie and I observed the same fact when working by the electrical method. However, when the radium is enclosed in a glass tube and placed at a distance from the condenser, which is itself enclosed in a thin aluminium box, it becomes a matter of indifference whether the screen be placed against the source or against the condenser; the current obtained is the same in both cases.