ARTICLES 577 



absorption like this goes with the excitation of the characteristic 

 X-rays of the absorber. Hence the inference mentioned above. 

 Moreover, it appeared that once the frequency of the exciting 

 ray passed this critical limit, all the lines of the K series were 

 excited at once ; the relative intensities of the lines have 

 always been the same in all experiments in which they 

 have appeared. 



These conclusions have been made very clear, mainly by 

 the work of Duane, Webster, Hull and others. In a special 

 experiment by Webster and Hull, described by Duane and 

 Kang-Fu Hu in the Physical Review, June 191 8, the wave- 

 lengths of the K series of rhodium were found to be, in three 

 separate determinations, as follows : 



The critical wave-length which is just short enough to be 

 highly absorbed by rhodium was found by arranging for a 

 continuous spectrum in this region to be given by an X-ray 

 bulb having a tungsten anticathode, and placing a screen of 

 a rhodium salt in the path of the rays. It was then found that 

 absorption set in at 0-5330, which is just shorter than the wave- 

 length of the 7 line. 



The critical frequency is extremely definite ; and as Dr. 

 Duane has pointed out, it is probably more representative of 

 the element to which it belongs than any of the radiations of 

 the K series, since it is unique, whereas the K series has four 

 different members at least. Blake and Duane have therefore 

 made a number of determinations of the critical frequencies 

 for various elements, some of which are given in the Physical 

 Reviezv, December 191 7 and June 191 8. With these are given 

 some previous results of similar character due to De Broglie 

 and Wagner. See the accompanying table (IV.) 



