Cause of the Structure of Spectra. 255' 



where v is the constant difference of the pairs ; and similarly 

 in the case of threes, the two other series art) given by 

 increasing n by v x and v 2 the constant differences in the threes, 

 as in the Mg series given at the beginning of Section 2. 

 By d berg connects the values o£ v or v 1 and v 2 for different 

 elements by the law that in each group of elements the value 

 of v increases in a somewhat quicker proportion than the 

 square of the atomic weight. It will be shown in the next 

 section that the relation between the values of v for different 

 elements of a group is purely numerical and not directly 

 connected with atomic weights. 



Series are divided into three classes — Diffuse, Sharp, and 

 Principal. The Diffuse and Sharp series consist of pairs or 

 threes of series of the sort just mentioned as specifiable by 

 means of one series and the common difference v or the 

 common differences v x and v. 2 , the adjectives Diffuse and Sharp 

 describing the appearances of the lines in the two classes. In 

 the Principal series, which have hitherto been clearly made 

 out only in the alkali metals, the lines are associated in pairs, 

 so that there are two principal series running side by 

 side, but the differences of their corresponding members are 

 no longer constant, the one series cannot be derived from the 

 other by adding a constant to n but by subtracting a constant 

 from fx. It may happen, as is probably the case with hydrogen 

 and lithium, that v and the constant to be added to \x become 

 so small that the usual spectroscopic appliances do not resolve 

 the pairs of lines, that is to say that the two diffuse series may 

 coincide and likewise the two principal series. 



The series can be conveniently named in the following 

 manner : — The Diffuse or Sharp scries which contains n is 

 the first Diffuse or Sharp series, that which contains n Q + v or 

 >i + v 1 the second, and that containing n + v 2 the third. 



Amongst Principal series, the stronger of the two, which is 

 also the more refrangible, is called the first, and the other 

 the second. These classifications are expressed by Rydberg 

 in a convenient notation which can be most easily explained 

 by an example or two. X(D 1 7J means that line for which 

 111 = 1 in the first Diffuse series of the element X; Y(S 2 4) 

 means that line for which ??< = 4 in the second Sharp series of 

 the element Y, and so on. 



Between the Diffuse and Sharp series of the same order 

 (first, second, or third) there is always this relation, namely, 

 that they have the same value of n , so that they differ only 

 in their value of /x, Rydberg assuming that a single value 

 of B holds not only for all the series of a given element, but 

 also for all elements. The>e facts can be conveniently 



