sOA/E CONTEMPORARY .IIH-.IXCr.S l\ I'liysiCS-lX (M 



this arliik- was dcvotwl. For an alotn, when initially in its normal 

 stati- and properly stinuilatetl, is aMe to rireixo enorRy in certain 

 definite measurable amc)unts, and to retain it for a while; and this is 

 tantamount to sayinjj that each atom may exist for a while in one or 

 aiiotiier of certain states distinct from the norma! state, in each of 

 which it pt)ssesses a certain distincti\e amount of extra enerjjv. 

 riuis a helium atom may recei\e 1!).75 equivalent volts of cnerjjy 

 from an impinging electron, no less and (within certain limits) no 

 more; and this is tantamount to sayinjj that a helium atom may exist, 

 not onU" in its normal state but also transiently in an abnormal state 

 in which its energy is greater by 19.75 equivalent volts than in the 

 normal state. The atom-model for each element must therefore be 

 designctl to be definite in each of several distinct and interchangeable 

 states, and not in one only. 



The energ\'-\-alues of some few of these stationary states are de- 

 terminable directly; but most of them (and they are very numerous) 

 are deduced from spectra. The spectrum of an element is the family 

 of radiations of various frequencies which it emits when it is in the 

 gaseous state. These arc commonly ascribed to the individual atoms. 

 The first task of the spectroscopist is to measure these frequencies; 

 his second, to classify them. In certain spectra his task of classifi- 

 cation is easy, for there is a natural arrangement of the spectrum 

 lines which "leaps to the eye." This is an arrangement of lines in 

 one or several converging series, like those of which there were photo- 

 graphs of the First Part of the article. Let me represent by 



the frec|uencies of the consecuti\e lines of a series, and by i/;„„ the 

 fre(|uency of the series-limit upon which they converge. Now the 

 frcciucncies of the various lines may be described by a formula 



Vi = Vlim—Ji (1) 



in which vi is expressed as the difference between two terms. The 

 term /, varies from one line to the next; and in some instances this 

 function /, is algebraically of an extreme simplicity, just the sort of 

 a simple elegance which is apt to suggest that the formula has an 

 inward physical meaning. Also one and the same term may figure 

 in the formulae for lines belonging to different series, a fact w'hich 

 enhances the feeling that the terms are physically "real." Thus 

 the spectroscopist seeks "terms" wherein' to classify the lines of a 

 spectrum; and the analysis of a spectrum leads to the measurement 

 of a multitude of terms. 



