On X-Ecujs and Scattered X-Rays. 289 
pulse. During the passage of an unpolarized primary beam, 
these accelerations are uniformly distributed in direction m 
a plane perpendicular to that of propagation of the beam. 
Now the electric intensity at a point P in one of the 
/» • /j 
resulting secondary pulses is expressed by ^^ , where 
e is the charge and f the acceleration of the electron, 
r the distance from the electron to the point P, and 6 the 
angle which the line joining the electron to the point P makes 
with the direction of acceleration. 
If P is on the line of propagation of the primary pulse 
over the electron 0= ^, and the intensity =^. 
If P is in the plane through the electron perpendicular to 
the direction of primary propagation, 6 varies uniformly 
from to 7r for the primary pulses. 
The intensities of radiation then in these two directions 
are proportional to 
e 2 f 2 e 2 f 2 sin 2 6 
X^r and X — — ^ respectively. 
A\ hen the primar}^ beam is unpolarized, the average value of 
sin 2 # = i, consequently the intensity of secondary radiation 
in the direction of propagation of the primary is double that 
in a direction at right angles. 
On the neutral pair hypothesis it is supposed that a pair 
which strikes a substance consisting of light atoms is liable 
to be taken up only by an atom revolving in the same plane. 
It is sometimes ejected again, and its subsequent rotation and 
translation continue to take place in the one plane ; there- 
fore all pairs ejected in any direction at right angles to that 
of propagation of the primary rotate in the plane of primary 
and secondary propagations. A tertiary beam is therefore 
strongest when in the same plane. This is Bragg's explana- 
tion of the polarization of the secondary beam as shown by 
the tertiary beams. 
The results of experiments described below, however, show 
that this cannot be the true explanation of the polarization 
effect ; neither is the theory capable of explaining the partial 
polarization of the primary. It is easily seen that on 
this hypothesis all the possible planes of rotation of pairs in 
a primary beam contain the direction of propagation of that 
beam, and therefore that any pair has a chance of being 
