The fact that a substance through which Röntgen rays from a focus tube are passing becomes itself a source of secondary Röntgen rays has long- been known. The most probable explanation was given by Prof. Sir J. J. Thomson. If a Röntgen pulse is due to the acceleration of a charged electron, then if the electrons in the atom are free to move under the action of the electromagnetic forces in the wave front of the primary Röntgen pulse, their motion will be accelerated during the passage of the latter through the atom, and they will themselves become sources of secondary Röntgen radiation. Considering only a single electron, the intensity of the secondary radiation at any angle α with the direction of motion will be proportional to sin
2
α
. If the primary beam is unpolarised, the motion of the electron may have any direction in the plane at right angles to the primary beam. The intensity of the scattered radiation in the direction
θ
with the primary beam is thus the mean of all the values of sin
2
α
for that direction. It can easily be shown that this is proportional to 1 + cos
2
θ
. If I'
θ
is the intensity of the scattered radiation in the direction
θ
, we thus have I'
θ
= I'
π
/2
(1 + cos
2
θ
).
On the distribution of the scattered Röntgen radiation
