Notices respecting New Books. 1133 



edition, but to supplement the book by a series of monographs 

 dealing with the branches of the subject which have been most 

 affected by the work of the past nine years. The modern electrical 

 theory of spectra has been entirely developed during this period 

 so that Dr. Campbell has naturally decided to devote his first 

 monograph to that subject. There were fourteen chapters in his 

 book ; consequently the book before us is entitled Modern 

 Electrical Theory, Chapter X\ T . 



Dr. Campbell's book is really needed, and he has acquitted 

 himself well of the very difficult task before him — to describe in 

 some hundred pages the experimentally established regularities of 

 series spectra, and the explanation of them on the lines first in- 

 dicated by Bohr, when he showed how the series formulae for 

 hydrogen and helium could be derived by an application of the 

 quantum theory to Rutherford's nucleus atom. 



Bohr's fundamental theory of stable non-radiating orbits is 

 well-known to readers of this magazine, where the papers de- 

 scribing it were first published; it has been considerablv extended 

 by Sommerf eld, Kossel , and especially by Bohr himself. Dr. Camp- 

 bell begins by describing briefly the properties of series, laying 

 particular stress on the Principle of Combination, which, of course 

 finds so simple an explanation on Bohr's theory, and indicating 

 the bearing of the ionization potential. After having outlined, 

 then, the fundamental facts which call for explanation he gives an 

 account of Bohr's early theory, with circular orbits, in all its 

 applications. He next deals with conditionally periodic orbits the 

 Zeeman and the Stark effect, and Sommerf eld's explanation of the 

 fine structure of the hydrogen and helium lines by applying the 

 relativity correction to the motion in the orbits. After this he 

 passes to the question of intensity of the lines, and treats of the 

 correspondence principle, which has assumed such great importance 

 in Bohr's latest work, which had been only hinted at when the 

 book was published. The book closes with a brief discussion of 

 band spectra, the study of which may be anticipated to give im- 

 portant results in the near future. 



In general the treatment is excellent. The discussion of the 

 experimental facts might have been a little fuller, but the reader 

 seeking more information can now turn to Professor Fowler's 

 ' Report on Series in Line Spectra,' which has just appeared, and 

 should be read in conjunction with Dr. Campbell's book. The 

 correspondence principle is not as clearly expounded as could be 

 wished, which is regrettable in view of its increasing application. 

 AVe note with sorrow a large number of misprints, of which 

 perhaps, the most serious and inexplicable is 6*17 x 10 4 printed 

 instead of 8'42 x ] 4 as the frequency for mercury vapour on p. 24. 

 A statement that there is only one electron in the inner rino- i s an 

 obvious slip. The positive merits of the book undoubtedly out- 

 weigh very heavily any minor defects, and if it does not achieve 



