RECENT ADVANCES IN SCIENCE 535 



Bigourdan, G., Observations des Nebuleuses et d'Amas 

 Stellaires (S vols., Publications of the Paris Observatory, 191 8). 

 These volumes contain in a collected form the whole of 

 Bigourdan's important series of observations of nebulae and 

 clusters, which were previously to be found only in a scattered 

 form throughout many publications. 



Aitken, R. G., The Binary Stars (pp. xiv + 316, New York, 

 191 8). Dr. Aitken is one of the foremost of living double-star 

 observers. This volume, which is one of the Semi-Centennial 

 Publications issued by the University of California, should be 

 in the hands of every double-star observer. 



PHYSICS. By James Rice, M.A., University, Liverpool. 



The Origin of Spectra. — This subject is discussed in the Guthrie 

 Lecture delivered by Professor McLennan of Toronto Uni- 

 versity and printed in the Proc. Phys. Soc. vol. xxxi. part i. 

 December 191 8. In this address certain experimental work 

 carried out by Prof. McLennan and his pupils together with 

 similar work carried out by other observers is summarised 

 and brought under review. References are given in the re- 

 print of the lecture and also in two papers contributed to the 

 Phil. Mag. December 191 8, by McLennan and two co-workers. 

 One of these papers in fact discusses more briefly the points 

 raised in the lecture. 



The gist of the address is an attempt to show the connec- 

 tion between the fundamental frequencies of the series of lines 

 exhibited in the spectra of various elements and the ionisation 

 potentials of those elements, and the bearing which this con- 

 nection has on the problem of atomic structure, especially 

 Bohr's suggested solution of that problem. To follow the 

 address the reader is assumed to be aware, in outline at least, 

 of the work which has been done by Balmer, Rydberg, Kayser, 

 Runge, Ritz and others on the discovery of formulae which 

 enable one to calculate all the lines in a given spectral series 

 from the knowledge of a few constants. Perhaps the best 

 known of these is the formula of Balmer which expresses the 

 lines of the hydrogen spectrum ; it is 



AT 



(*-*)■ 



Here v represents the " wave-number," that is, the frequency 

 divided by the velocity of light, or the number of waves in 



