S!OME CONTEMPOR.IRV .IPl.tXCFS IX PHYSICS-VIll 4.M 



lilies with another atom or with an electron. Otherwise, it seems 

 that if the atom cannot radiate the enern>' liiieratetl in a transition, 

 the transition itself cannot hap(X'n at all. If, therefore, the line ror- 

 resjiontlini; to an arrow is niissin^, the transition corresponding to 

 the arrow must he inhit)ile<l by some agency as yet imknown. M.iiu' 

 transitions must he inhihiteii, for many lines are missing. 



These missing lines are precious to the student of spectra and to 

 the .irchitect of atom-models. \Vhate\er explanation is devised for 

 the stationar>- states must include a reason for the occurrence of 

 some transitions and the non-occurrence of some others. This is 

 gcMxl rather than had fortune, since if such a reason is demanded, it 

 may he found in one and not in another of two competing theories 

 which otherwise wouUl stand on an equal footing; the missing lines 

 may even suggest a theory. At all events they suggest a system of 

 classitication; and, while the hardcne<I theorizer may regard a system 

 of classitication as merely the forerunner of a theory, a theory is itself 

 often nothing more than a classification stated in the language of an 

 artificial analogy. It is, in fact, possible to arrange the stationary 

 states, not in a single column as in Figs. 1 or 5, but in several as in 

 Fig. G; this arrangement being so contrived, that any transition can be 

 identified in a moment as belonging among those which occur, or 

 among those which are missing, whichever its case may be. 



The mere fact that such an arrangement can be contrived shows 

 that the missing lines are not distributed at random, but subject to 

 some sort of a rule. Such rules are known as principles of selection. 

 The missing lines are commonly called verboten lines by the German 

 physicists, [xissihly because that was the most conspicuous word 

 in the otTicial German language before the war. It is not a happily- 

 chosen word, neither are the English equivalents "forbidden" and 

 "prohibited"; since while we know that the lines are missing, we do 

 not definiteK' know what circumstance is responsible; and, whatever 

 that circumstance may be, it is highly unconventional for a ph>'sicist 

 to say that it "forbids" the lines. The same objection applies with 

 extra force to the phrase "forbidden by the selection-principle". 

 It is much better to accept the fact that certain lines are missing as 

 a fact of experience, and the selection-principles as rules of experience 

 whereby the facts are codified. 



£ 4- The Families of Stationary States (for Other Atoms than Hydrogen) 



There is a far-reaching contrast between the spectra of all atoms 

 hut hydrogen and ionized helium, on the one hand, and the spectra 

 of these two atoms on the other. The selection-principles at first 



