THEORY AND FORMER INVESTIGATIONS. ^^ * 



I. General. 



It is not the purpose of the author to give here in any detail the development of the theory of the 

 Zeeman effect or to summarize at length the many investigations which have led to the present state 

 of knowledge regarding the phenomenon. Several such accounts have appeared in publications which 

 are usually accessible. Among these may be mentioned the memoir of Cotton (r)* (1899), the chapter 

 by Runge in Kayser's Handbiich der Spectroscopie (2) (1902), the detailed discussion by Voigt (3) (1908) 

 in connection with the related optical phenomena, and the brief treatment by Lorentz (4) (1909) in his 

 Columbia Lectures. Of these the second is by far the most complete, covering fully the historical devel- 

 opment, methods of investigation, and the theory and spectroscopic results contained in the literature 

 up to that time. For the purposes of the present paper, we shall consider the points in the theory which 

 apply closely to the results of this investigation, and summarize the work of other investigators in so 

 far as their results relate direct!}- to those of the present research. 



The later work on the Zeeman phenomenon has been concerned largely with the study of complex and 

 unusual types of separation. It was shown during the earlier investigations by Zeeman (5) , Michelson (6), 

 Preston (7), Cornu (8), Becquerel and Deslandres (9), (10), Ames, Earhart and Reese (n), Reese (12) , and 

 Kent (13) that a large proportion of the spectrum hnes of any of the elements that have been examined 

 are split into more than three components. This involved an extension of the original theory of Lorentz, 

 which satisfactorily explained the triplet separation, in which two components are given by the light 

 vibrations in a plane perpendicular to the lines of magnetic force, these showing respectively a right- 

 handed and a left-handed circular polarization, and a central component by the light vibrations paral- 

 lel to the magnetic force-lines. Since the phenomenon in its simplest form justified taking the electron 

 theory as the basis of all conceptions of the action of the magnetic field upon spectra, a series of investi- 

 gations, among which those of Lorentz (14), Larmor (15), Voigt (16), and Robb (17) may be mentioned, have 

 greatly extended the mathematical theory, both for radiation in general and for the explanation of the 

 more complex forms of magnetic separation. Voigt and Robb have based their theory on the idea of 

 mutually connected systems of electrons, and have thus been able to account for many of the more com- 

 plicated types of Zeeman separation. However, both the nature of the connections and the way the 

 magnetic field efTects such systems are but imperfectly explained. 



The proportionality of separation of components to field-strength has been worked on by Reese (i'), 

 Kent (13), Runge and Paschen (i8),Farber {19), Weiss and Cotton (2o),Paschen (21), and Stettenheimer (22), 

 and established to a very close approximation. The law enunciated by Preston (23 1 that the character of 

 separation and chstance between components (measured in terms of change of vibration frequency) is the 

 same for corresponding lines in the series of Balmer, Rydberg, and Kayser and Runge has been investigated 

 by Reese (12), Kent (13), Runge and Paschen (24), Runge and Precht (25), Miller (26), and Lohmann (2:). 

 The last two have found some exceptions, though Runge and Paschen observed very close agreement for 

 the series lines of a number of elements. This relation has frequently been used, recently by Moore (28), 

 in an attempt to find series among spectra containing many lines. 



There has been considerable work in recent years on the commensurability of the separations of 

 spectrum lines, that is, on the existence of a fundamental interval of which the separations of all com- 

 plex lines are multiples, and on the extent to wliich this applies to the separations of triplets in which 



* Numbers in parentheses indicate references to the Uterature on p. 65. 



