98 NATURE OF THE RADIATIONS [CH. 
while the rays nearly parallel to the plate strike the plate at 
nearly grazing incidence. The rays, inclined to the direction of 
the field, describe spirals and produce effects on an axis parallel 
to the field passing through the source. In consequence of this, 
any opaque screen placed in the path of the rays has its shadow 
thrown near the edge of the photographic plate. 
71. Complexity of the rays. The deviable rays from 
radium are complex, .e. they are composed of a flight of particles 
projected with a wide range of velocity. In a magnetic field every 
ray describes a path, of which the radius of curvature is directly 
proportional to the velocity of projection. The complexity of 
the radiation has been very clearly shown by Becquerel? in the 
following way. 
An uncovered photographic plate, with the film upwards, was 
placed horizontally in the horizontal uniform magnetic field of 
an electro-magnet. A small, open, lead box, contamimg the 
radio-active matter, was placed in the centre of the field, on 
the photographic plate. The light, due to the phosphorescence 
of the radio-active matter, therefore, could not reach the plate. 
The whole apparatus was placed in a dark room. The impression 
on the plate takes the form of a large, diffuse, but contimuous 
band, elliptic in shape, produced on one side of the plate. 
Such an impression is to be expected if the rays are sent out 
in all directions, even if their velocities of projection are the same, 
for it can readily be shown theoretically, that the path of the rays 
is confined within an ellipse whose minor axis, which is at right 
angles to the field, is equal to 2, and whose major axis is equal 
to wR. If, however, the active matter is placed in the bottom of a 
deep lead cylinder of smal! diameter, the rays have practically all 
the same direction of projection, and in that case each part of the 
plate is acted on by rays of a definite curvature. 
In this case also, a diffuse impression is observed on the plate, 
giving, so to speak, a continuous spectrum of the rays and showing 
that the radiation is composed of rays of widely different curvatures. 
Fig. 22 shows a photograph of this kind, obtained by Becquerel, 
with strips of paper, aluminium, and platinum placed on the plate. 
1 C. R. 130, pp. 206, 372, 810, 979. 1900. 
