544 SCIENCE PROGRESS 



transformation corresponds to a rotation in a four-dimensional space, the fourth 

 dimension being a modified time. He gives an exposition of Minkowski's matrices, 

 and adds a most interesting and original account of a quaternionic method of 

 dealing with the " four vectors " involved, demonstrating the convenience of 

 quaternions in this case. Some of the most remarkable results of the relativity 

 theory are contained in the chapter on the composition of velocities. The theory 

 does not modify by appreciable terms our classical Newtonian formulae as long as 

 the ratios of the velocities in question to that of light are small, but when these 

 ratios approach unity entirely new formulas must be used for compounding 

 velocities, which are such that if an observer measures the velocities in opposite 

 directions of two different bodies relative to himself, no matter how near the 

 velocity of light these may be, the velocity of the one body as measured by an 

 observer on the second will still be less than that of light. In fact, a relative 

 velocity greater than that of light is impossible, in so far as it cannot be dealt 

 with by the relativity theory, which is based on the use of light signals. 



There is much more of the greatest interest in the book, to which reference 

 cannot be made here for want of space. The vector methods exposed in the 

 author's book on vector mechanics, recently reviewed in these columns, are used 

 with great lucidity, and the construction and style of the book make it very 

 attractive to read. Far more comprehensible, if less comprehensive, than Dr. 

 Laue's book, which has only appeared in the original German, Dr. Silberstein's 

 work is, we think, the badly needed book which is to make the theory of relativity 

 accessible to the large number of physicists who wish to obtain a clear idea of its 

 scope and achievements without devoting themselves exclusively to this subject. 



E. N. da C. A. 



Photo-Electricity. By A. L. Hughes, D.Sc, B.A. [Pp. viii + 144.] (Cam- 

 bridge University Press. Price 6s. net.) 



SINCE Hertz in 1887 showed that a spark passed between metallic terminals more 

 easily when the negative terminal was illuminated, and Hallwachs supplemented 

 this observation by the discovery that a negatively charged body rapidly loses its 

 charge when illuminated with ultra-violet light, the study of the liberation of 

 electricity by light — photo-electricity — has received the attention of a very large 

 body of physicists, including some of the most distinguished names of the period, 

 and has afforded one of the most baffling problems of modern electrical science. 

 A short time ago Dr. Allen summarised the work done in this region in a book 

 reviewed in these columns, and there has recently appeared a book by Dr. Hughes, 

 now under review, devoted to the same subject. 



Dr. Hughes has himself carried out important investigations in this field, and 

 brings to his task a very complete knowledge of the literature of the subject and 

 of the peculiar difficulties of its problems. This is fortunate, for without an 

 intimate acquaintance with the technique of the experiments it is hopeless to 

 attempt a clear exposition in the face of so much indeterminate and contradictory 

 work as has been carried out in photo-electricity. He considers first the ionisa- 

 tion of gases and vapours by ultra-violet light, and rightly points out that the 

 interpretation of results would be much easier in this case than in the case of 

 solids, if only the experimental difficulties connected with the extremely short 

 wave-lengths involved could be overcome. This is perhaps the most promising 

 field for obtaining definite results. This chapter gives an excellent account of 

 the photo-electric effect in gases, which was badly needed in English. Then 



