Screens employed. Thickness. │ Current observed. M.m. │ Aluminium 0·01 │ 17·9 Brass 0·005 │ 〃 ───────────────────────────────────────────────┼─────────────────────── Brass 0·005 │ 6·7 Aluminium 0·01 │ 〃 ───────────────────────────────────────────────┼─────────────────────── Aluminium 0·01 │150 Tin 0·005 │ 〃 ───────────────────────────────────────────────┼─────────────────────── Tin 0·005 │125 Aluminium 0·01 │ 〃 ───────────────────────────────────────────────┼─────────────────────── Tin 0·005 │ 13·9 Brass 0·005 │ 〃 ───────────────────────────────────────────────┼─────────────────────── Brass 0·005 │ 4·4 Tin 0·005 │ 〃 The results obtained prove that the radiation is modified in passing through a solid screen. This conclusion accords with the experiments in which, of two similar superposed metallic screens, the first is less absorbent than the second. From this it is probable that the transforming action of a screen increases with the distance of the screen from the source. This fact has not been verified, and the nature of the transformation has not been studied in detail. I repeated the same experiments with a very active salt of radium; the result was negative. I only observed insignificant variations in the intensity of the radiation transmitted with interchange of the order of the screens. The following systems of screens were experimented with:— Aluminium, thickness 0·55 m.m. Aluminium, thickness 0·55 m.m. Aluminium, thickness 0·55 m.m. Aluminium, thickness 1·07 m.m. Aluminium, thickness 0·55 m.m. Aluminium, thickness 1·07 m.m. Aluminium, thickness 0·15 m.m. Aluminium, thickness 0·15 m.m. Aluminium, thickness 0·15 m.m. Platinum, thickness 0·01 m.m. Lead, thickness 0·1 m.m. Tin, thickness 0·005 m.m. Copper, thickness 0·05 m.m. Brass, thickness 0·005 m.m. Brass, thickness 0·005 m.m. Platinum, thickness 0·01 m.m. Zinc, thickness 0·05 m.m. Lead, thickness 0·1 m.m. The system lead-aluminium was slightly more opaque than the system aluminium-lead, but the difference was not great.