298 Dr. C. G. Barkla and Mr. G. H. Martyn on the 



radiations as abscissae and the photographic effect (measured 

 as indicated) as ordinates, the features of the curve become 

 crowded at one end of the figure. But though no direct 

 determinations of the wave-lengths of the fluorescent X-radi- 

 ations have hitherto been possible, if we extend the con- 

 clusions arrived at with regard to ultra-violet light to these 

 Rontgen radiations we may arrive at the wave-lengths in- 

 directly. As to the essential ideutity of ultra-violet light 

 and X-rays there can be no doubt, and there is no reason to 

 even suspect that the Law connecting frequency of light and 

 the velocity of emission of electrons by substances exposed 

 to that light is not applicable also to X-rays. There is much 

 evidence of the applicability, and, as far as we are aware,, 

 none against. We shall, at any rate, accept it provisionally.. 

 The work of Whiddington has shown that the maximum 

 velocity of the electrons set free from a plate by a fluorescent 

 radiation (series K) from an element of atomic weight w is 

 about ivX 10 8 cm. per sec* But if this is the whole energy 

 of a quantum on Planck's theory 



^mv 2 = 7m, 



where m and v are the mass and velocity of the electron,. 

 h is Planck's unit 6*55 x 10~ 27 5 and n is the frequency of 

 light. We can thus calculate the frequency and hence the 

 wave-length. 



43.7 x 1010 

 The wave-length X is found to be — 



v 2 



But v= iv xlO 8 . 



Hence, assuming the whole energy of a quantum goes 



into an electron, 



. -437 xlO" 4 ,. , 



\ = about 5 centimetre. 



w 



If, however, we accept the experimental results of A. L. 



Hughes, hmv 2 = h'n t, where li is about f- of Planck's con- 



1 n K 1 -38X10" 4 

 stant, we get a value tor a, or about 5 cm. 



Accepting this experimental value, — Hughes suggests that 



* This is probably the velocity of all electrons when just freed from 

 the atom. 



t The relation found by Hughes is written jmv 2 — h'n— E , but E is 



negligible in this case. The value of h' was found to vary slightly with 

 the substance emitting the electrons. The average of thesevalues is 

 taken above, 



