22 SCIENCE PROGRESS 



Planck's " h " constant by means of X-rays using the quantum 



relation 



Ve = hn s 



where e is the elementary charge and n the frequency of 

 X-rays which are produced by the voltage V. The value 

 agrees closely with those obtained by other experimenters 

 using the same method, and also those obtained by Millikan 

 and others using the photoelectric effect. The feature of 

 the method is the design of a new type of X-ray spectrometer, 

 resembling very closely the ordinary optical spectrometer in 

 several of its details. The second paper by these authors 

 deals with the more accurate determination of the frequencies 

 at which marked increases in the absorption of X-rays by a 

 chemical element takes place. Uhler and Cooksey give an 

 account in the same number of this journal of experiments 

 to determine with considerable accuracy the wave-lengths for 

 the K series of the X-ray spectrum of gallium. They also 

 use a spectrometer different from the usual type ; for as their 

 work proceeded, the difficulties and sources of error inherent 

 in the usual method became so prominent as to cause the 

 first named of the authors to make an analytical study of the 

 general problem of determining glancing angles. The results 

 of this study are presented in the January 191 8 number of 

 the Phys. Rev., and by them it became possible to subject 

 the old method and the new one to very thorough practical 

 tests. Like Messrs. Blake and Duane, these collaborators 

 designed a spectrometer resembling in some details the optical 

 spectrometer. 



In the April number of the Proc. Roy. Soc, Prof. Richardson 

 discusses the photoelectric action of X-rays from the point 

 of view of the quantum theory, basing his remarks on the 

 equation 



\mv i — hn — w 



where the left-hand side represents the maximum kinetic 

 energy of the liberated electrons, h is Planck's constant, n 

 the frequency of the exciting radiation, and w a constant 

 which measures the work necessary for an electron to escape 

 from a substance, its value being characteristic of the material 

 under consideration. The paper is by no means easy reading ; 

 but the author advances the view that a survey of the experi- 



