I.] PRINCIPLES AND METHODS. 11 



out the lines into what are termed " series," that is lines numerically 

 related to each other. 



Messrs. Runge and Paschen* showed, in 1890, that the spectra of 

 lithium, sodium, and potassium were the summation of the spectra of 

 various " series." Later they have shown that the same is true in the 

 case of the cleveite gases. 



Violet. Red 



l"iG."ll. The series in the cleveite gases. 



A " series " of spectral lines may be defined as a sequence of lines, 

 the intensity of which decreases with the wave-length, and the wave- 

 number or wave-frequency of which may be determined by the 

 formula 



A + BM 2 + G//i 4 , 



where n represents the integers from three upwards, and the constants 

 A, B, and C are determined for each element separately. The shorter 

 the wave-lengths the greater number of waves there will be in a given 

 length ; hence the wave-frequency varies inversely as the wave-length. 



The fact that lines must close up to one another, as the violet end 

 of the spectrum is reached, indicates that the character of a " series " 

 is best brought under notice in the ultra-violet end of the spectrum. 

 In the visible part of the spectrum the lines forming " series " are too 

 far apart to be recognised as belonging to a series. 



The accompanying diagram (Fig. 11) shows how the apparently 

 irregular lines observed in the spectra of the cleveite gases can be 

 arranged into the most exquisite order when the six series of lines which 

 build up the spectra are shown separately. 



Some of these series are composed of triplets and some of doublets 

 instead of single lines. 



* AWi. k. AJcad. Wist., Berlin, 1890. 



