REVIEWS 701 



average reader, however, many of these references may seem involved and 

 difficult to understand ; this is partly because certain facts about the solar system 

 and the stars which are now common knowledge were unknown in Dante's time, 

 and to understand many of his references it is necessary to view them from his 

 view-point ; again, the average reader of to-day is quite unfamiliar with the face 

 of the sky and with the motions of the sun, moon, and planets in the different 

 constellations, so that Dante's many picturesque methods of indicating a season 

 or length of time convey absolutely no meaning. 



The purpose of Mrs. Evershed's book is to remove these difficulties and to 

 enable Dante's astronomical allusions to be understood. The book is divided into 

 two parts ; in the first is given an account of the elementary facts of astronomy 

 which will help the non-astronomical reader to understand what follows ; a sketch 

 is then given of the various attempts made to explain these movements from the 

 earliest times until the time of Dante. This sketch, which occupies about 200 

 pages, is admirably and concisely done, and forms a valuable summary of the 

 early history of astronomy ; for the benefit of the general reader all unnecessary 

 technicalities are avoided. In the second part is given an account of the popular 

 astronomy in Italy in Dante's time and of the books on astronomy which he 

 probably used ; the thoroughness of his knowledge is illustrated. This is followed 

 by a careful discussion of Dante's numerous references and by a critical examination 

 of some of the difficult passages whose interpretation is more or less uncertain. 

 The aim throughout has been, however, not so much to interpret the references 

 for the reader as to provide him with sufficient information to enable him intelli- 

 gently to interpret them for himself: in this aim Mrs. Evershed has been 

 eminently successful, and we can strongly recommend her charming book to all 

 students and readers of Dante. We may add that the illustrations throughout 

 are excellent and have been carefully thought out. 



H. S. J. 



PHYSICS 



The Emission of Electricity from Hot Bodies. By Prof. O. W. Richardson, 

 F.R.S. [Pp. vi + 304, with 35 diagrams. Monographs on Physics Series.] 

 (London : Longmans, Green & Co., 191 6. Price gs. net.) 



It is but seldom that a branch of science is so completely dominated by one man 

 as is the subject of this book by Prof. Richardson. Other workers have played 

 an honourable part, notably H. A. Wilson, Prof. Horton, and Irvine Langmuir; 

 but our knowledge of thermionics would be much scantier and very much less 

 precise than it is at present without the guidance that his work has provided. 

 The explanation is not far to seek, for the subject makes demands of no ordinary 

 character on the mathematical and physical knowledge of the investigator and on 

 his skill and patience in overcoming experimental difficulties of unusual severity. 

 The pioneer work has been done ; but many points still need elucidation, 

 and one of the most pleasing features of the book is the care with which the 

 author indicates, again and again, questions which should repay investigation. 

 It is a mine of suggestions for any one seeking an interesting line of research. 

 With the exception of the electrical properties of flames and certain technical 

 applications, the whole of the phenomena embraced by the title Thermionics 

 are dealt with, and very little, if any, work of importance has been omitted. 

 From the general point of view the interest still centres very largely on the 

 rival theories of the emission of the electrons ; as to whether it is to be 

 attributed to the purely thermal increase in their kinetic energy or to chemical 



