Theory of X Rays and Photoelectric Rays. 



537 



by X rays that we may neglect them, though it would not 

 complicate matters much if we did not. Neglecting q 1 and 

 q 2 , however, we have x/y = y/2c. If we consider the case 

 of the j3 rays ejected by X rays, which have a velocity 

 of the order 6 x 10 9 cm./sec, we see that for these rays 

 xjy = 1/10. Now suppose that a metallic film with its plane 

 perpendicular to the plane of the paper is represented in 

 section by AB (fig. 1), the train of waves being incident in 



Fig. 1. 



the direction of the arrow S. If x were zero the rays would 

 start out in all directions parallel to the plane AB, if the 

 train were unpolarized, or in two opposite directions if it were 

 plane polarized. Consider an electron which at D receives 

 a velocity y in the positive direction of y. Its chances of 

 deflexion in any direction by the first atom which it meets 

 may be represented in the valuable manner suggested by 

 Professor Bragg, by the radius vector of an egg-shaped 

 surface, the radius vector being drawn from the point 0. 

 This oval would obviously be symmetrical about the plane 

 AB, so there would be symmetry with respect to this plane 

 in the number and velocities of the j3 rays escaping on the 

 two sides of the film. If x is not zero, however, the de- 

 flexion oval will be symmetrical, not about the line AB, but 

 about a line GH or EF, according as y is positive or nega- 

 tive, the inclination 8 of these lines to AB being such that 

 tanO = xly. If dn is the number of electrons sent off from 

 an element ds at D per second- in the direction DH, the 

 fraction of these which are deflected within a given solid 

 angle d£l measured from will be proportional to rd£L, 

 where r is the radius vector corresponding to d£l, and con- 

 sequently the number passing through the plane through 

 parallel to AB (which is practically the same as the plane 

 AB) in the forward direction is to the number passing 



