PHOTONS AND ELECTRONS 21 



little or no use of the picture of a revolving electron, are likely enough to 

 speak or write of the ''VS orbit of mercury" instead of the "stationary 

 state of the mercury atom denoted by VS." There is, in fact, no com- 

 pelling reason why the practicing spectroscopist and the user of light 

 as a tool should not continue to employ both the word and the picture. 

 The important point is that the stationary states and their energy values 

 are now so fully verified as to have risen practically to the rank of facts of 

 experience, unaffected by alterations, in the models by which theoretical 

 physicists are accustomed to represent them. When the reader reahzes 

 this, he may safely designate a stationary state by such words as "orbit," 

 or "position of an electron," or "arrangement of the electrons," without 

 concerning himself unduly with the fact that some physicists use different 

 descriptive words or none at all. 



TERMS 

 Reverting to Balmer's formula: the frequency of any line of the 



r> D 



Balmer series is given as the difference j- 5, which we may write thus 



rr ft 



instead : 



-R /-R\ , , 



The reason for this algebraically correct but oddly appearing revision 

 is seen by inspecting the theoretical equivalent of Balmer's formula, 

 equation (i5). The energy values of the nth and the second orbits, 



respectively, are hRy, 1 A and }\R\\ — j) when reckoned from our 



former zero, viz.^ the energy of the normal state. The quantities —hR/n^ 

 and —hR/4L are the energy values of these orbits, reckoned from a new 

 zero greater by hR than the old one. From equation (13) we can divine 

 the meaning of this new zero. The quantity hR is hvy,^, the energy corre- 

 sponding to the limit of the Lyman series, therefore the energy just 

 sufficient to extract an electron from a hydrogen atom initially in the 

 normal state. In passing from the old zero of energy to a new zero greater 

 by hR, we have passed from the "normal state of the neutral atom" 

 to the "state of the ionized atom" as the new standard from which energy 

 values are to be reckoned. The quantity —hR/n- is the energy of the 

 hydrogen atom with the electron in its nth orbit, reckoned from the 

 energy of the ionized atom with its electron at infinity. 



The practical importance of this point is as follows: In spectroscopic 

 tables and elsewhere, the frequency of a spectrum line belonging to a series 

 is often written as the difference between two "terms," 



V = A - B = -B - (-A) {16) 



