SOME CONTEMPORARY ADVANCES IN PHYSICS— X 111 



say) the sodium atom. It takes a certain amount of faith to go about 

 the business of designing such a central field; for the model imagined 

 for the sodium atom involves ten electrons around the nucleus in 

 addition to the one "valence" electron for the benefit of which the 

 field is being devised; and one might expect these ten electrons to be 

 rushing around the nucleus in uncoordinated and non-recurring paths, 

 never at any two instants similarly placed and similarly moving, 

 never at any two instants exerting the same influence upon the valence- 

 electron. Yet the Stationary States of the sodium atom are as 

 sharply defined as those of the hydrogen atom; and this may be 

 thought to mean that the ten electrons of the kernel are constrained 

 to a unity and a fixed relationship, like that of the members of a 

 machine if not like that of the parts of a rigid body, which translates 

 itself into an influence upon the valence-electron not unlike that of a 

 central field. 



At all events, several physicists working independently in various 

 nations have taken the not inconsiderable trouble of devising central 

 fields to fulfil the condition required; and they appear to have ac- 

 chieved a respectable success. It is not easy to decide what this 

 success requires the rest of us to believe; perhaps it is formally possible 

 to devise a central field to account for any set of Stationary States; 

 I am not sure whether this question has been adequately examined. 

 Some have felt confident enough to say that the results show which of 

 the Stationary States correspond to orbits of the valence-electron 

 which "penetrate into the kernel" and which to orbits that remain 

 in all their circuit quite outside of the kernel. It is to be hoped that 

 this problem will become clearer in the next few years. At this point 

 I will add only, that the orbits traced for the valence-electron are 

 rosette orbits in which the precession is very rapid, so that consecutive 

 loops of a rosette are inclined at a considerable angle to one another. 

 In the model for the hydrogen atom, the consecuti^•e loops of a rosette 

 orbit lie so close together as to be indistinguishable when drawn to 

 scale on an ordinary sheet of paper (the separation between them 

 was much exaggerated in Fig. 3 of the Second Part of this article). 

 In these atom-models, the orbit looks rather as if it were drawn along 

 the edges of the blades of an electric fan. 



Q. Interpretation of the Optical Spectra 

 OF Other Elements 



As soon as we step from the first column of the Periodic Table into 

 the second, the obstacles to such a theory as we have hitherto tried 

 to hold are gravely increased. There is e\'idence of se\-eral kinds 



