magnetic field on a pencil of rays from a radio-active substance giving out the three kinds of rays is very well illustrated in Fig. 222.
Some radium is placed in the bottom of a narrow cylindrical lead vessel R. A narrow pencil of rays consisting of α, β, and γ rays escapes from the opening. If a strong uniform magnetic field is applied at right angles to the plane of the paper, and directed towards the paper, the three types of rays are separated from one another. The γ rays continue in a straight line without any deviation. The β rays are deflected to the right, describing circular orbits the radii of which vary within wide limits. If the photographic plate AC is placed under the radium vessel, the β rays produce a diffuse photographic impression on the right of the vessel R. The α rays are bent in the direction opposite to that of the β rays, and describe a portion of the arc of a circle of large radius, but they are rapidly absorbed after traversing a distance of a few centimetres from the vessel R. The amount of the deviation of the α rays compared with that of the β rays is much