OCTOBEE 23, 1914] 



SCIENCE 



597 



appearance of two books on tlie subject of 

 photo-electricity is a ■welcome response to a 

 very general demand. 



Dr. Allen's book is intermediate in char- 

 acter between a popular or semipopular pre- 

 sentation — such as would be suitable for the 

 reader with a general scientific interest, or for 

 workers in subjects related to physics — and a 

 detailed summary intended for the specialist. 

 After two introductory chapters, giving a very 

 brief general survey of the whole field and an 

 account of the work of the early experimenters, 

 the author takes up in greater detail such 

 subjects as the emission of electrons in a 

 vacuum, the velocity of the electrons emitted, 

 the photo-electric behavior of different sub- 

 stances, the influence on photo-electric phe- 

 nomena of gas-pressure, temperature and the 

 wave-length of the exciting light, and photo- 

 electric fatigue, to whose experimental study 

 Dr. Allen has himself contributed. Then fol- 

 low chapters on " Theories of Photo-electric 

 Action," " Fluorescence and Phosphorescence," 

 and " Photo-electrical Actions and Photog- 

 raphy." Since the topics treated in the last 

 two chapters, although probably related to the 

 subject of photo-electricity, are not to be re- 

 garded as essential parts of the general sub- 

 ject, the author makes no attempt at a com- 

 plete treatment of these topics. 



Dr. Allen makes no attempt to summarize 

 all the articles dealing with photo-electricity, 

 and has not given a complete bibliography. 

 The value of the book would have been 

 greatly increased if a complete list of articles 

 had been given. But even without this the 

 book is of great value. In discussing each 

 topic references are given to some of the more 

 important original articles, so that the reader 

 who wishes to go into the subject in detail will 

 be greatly helped. 



For the specialist I imagine that Dr. 

 Hughes's book will be found the more useful 

 of the two. It assumes considerable familiar- 

 ity on the part of the reader with the subject 

 and with related subjects, and goes into 

 greater detail in the critical discussion of the 

 results obtained by different observers. This 

 is especially true in the case of those phases 

 of the subject which are now attracting most 



attention. At the end of each chapter there 

 is a summary of results, and in many cases a 

 discussion of their theoretical bearing, which 

 will be found very useful. References to orig- 

 inal sources, although not complete, include 

 most of the more important articles. In the 

 main the ground covered is the same for the 

 two books. Dr. Hughes includes, however, a 

 chapter on the ionization of gases by ultra- 

 violet light, which is looked upon as one in- 

 stance of photo-electric action. There are also 

 short chapters on positive rays produced by 

 light, and on the sources of light used in 

 photo-electric experiments. On the other hand 

 the treatment of photo-electric fatigue is less 

 complete than in Dr. Allen's book, and the 

 subjects of luminescence and of photographic 

 action, to which Dr. Allen devotes two chap- 

 ters, are not taken up at all. 



In discussing the velocity of emission of 

 photo-electrons Dr. Hughes points out that 

 while it seems probable that a linear relation 

 exists between the maximum energy of the 

 photo-electrons and the frequency of the 

 exciting light, so that the retarding potential 

 necessary to prevent discharge is given by 

 V '=Tcn — Vg, the constant h has not been 

 found to have the value h/e which Einstein's 

 theory, based on the theory of quanta, would 

 lead us to expect. Since this chapter was 

 written Millikan's measurements on sodium 

 have not only established the linear relation 

 between V and n with greater exactness and 

 through a wider range than has been done by 

 any previous investigation but have also led 

 to a value of Tee which differs from h by con- 

 siderably less than the uncertainties in the 

 value of Planck's constant. This chapter of 

 Hughes's book would undoubtedly have been 

 considerably modified if it had been written 

 after the publication of Millikan's work. 

 Nevertheless the author's discussion retains 

 its value, for the results obtained by other ob- 

 servers still requires explanation. 



I am convinced that these books will be 

 found most useful, both by those who wish only 

 to be informed regarding recent progress in 

 the subject of photo-electricity and by those 

 who are engaged in investigation in this field. 

 Ernest Mereitt 



