566 SCIENCE PROGRESS 



The theoretical work which has been done on the subject is most attractively 

 exposed, the essential assumptions of each theory and its main consequences 

 being set out with brevity and great clearness. In the last chapter the origin of 

 the spectral series, and the complicated types of resolution of the lines in the 

 magnetic field, are considered from the theoretical standpoint, and an account 

 given of Ritz's theory of the series, and Lorentz's theories of the coupling of the 

 emission centres by the magnetic field, which, with Voigt's modifications, is capable 

 of accounting for the various types of resolution. The necessary assumptions, 

 however, appear most artificial, and it cannot be said that the present state of the 

 theory is altogether satisfactory. Ritz's theory of the Zeeman effect has been 

 shown to be unworkable by Voigt. Dr. Bohr's papers on the constitution of the 

 atom, which have appeared during the last few months, seem to supply a more 

 simple and inclusive theory of the spectral series, though it remains to be seen if 

 his atom can be induced to give the experimental Zeeman resolutions. 



The style and arrangement of the book are throughout admirable, and the 

 author has contrived to give a very clear account of mathematical theories with a 

 minimum of symbolic working. The many personal touches, such as the rather 

 pathetic account of the postponement of the work on the types of resolution of 

 lines of the same series for lack of suitable equipment, give an added attractive- 

 ness. It would be difficult to speak too highly of the general production of the 

 book, which contains a large number of the most beautiful photographs illustrating 

 the various effects. 



E. N. DA C. A. 



Principles and Methods of Geometrical Optics. By J. P. C. Southall. 

 [Pp. xxiii + 626.] Second Edition. (Macmillan & Co., 1913. Price 

 2 5 j. net.) 



The fact that Professor SouthalPs work on Geometrical Optics, first published in 

 1910, has already appeared in a second edition, shows that it has speedily won 

 the recognition it deserves as the best book on the subject in English. The 

 changes in the new edition are small ; the arrangement and pagination have been 

 left exactly as in the first edition, two appendices to the chapters on Refraction 

 of a Narrow Bundle of Rays through a System and on the Theory of Spherical 

 Aberration respectively having been bodily inserted on lettered, not numbered, 

 pages. These appendices are in the nature of further notes to the chapters ; the 

 second contains a detailed account of the calculation of the spherical errors of 

 a centred system by means of the Seidel formulae. 



To the English reader who wishes a full and clear account of the recent work 

 on geometrical optics, especially that of the Germans — we need only mention the 

 names of Abbe, Petzval, Seidel, and Czapski — to whom most of the recent advances 

 are due, the book can be unreservedly recommended. It covers the whole field 

 in a complete and satisfactory way, making full use of the most modern methods, 

 many of which are not to be found in any other text-book of which we know. 

 Especial attention is throughout devoted to the applications to the design of 

 modern optical instruments. 



It is to be hoped and expected that the book will do much to revive the study 

 of geometrical optics in England, which, although at present the Germans are its 

 undisputed masters, was the first home of the science. 



E. N. DA C. A. 



