THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 277 



of cleveite is not simple. In their view the mixture consists 

 of two constituents. 



This conclusion was arrived at from the following con- 

 siderations. " The wave-lengths A of the lines belonging to 

 the same series are always approximately connected by a 

 formula somewhat similar to Balmer's — 



i/X = A - B/;// 2 - C/m\ 



A determines the end of the series towards which the lines 

 approach for high values of m, but does not influence the 

 difference of wave-numbers of any two lines. B has nearly 

 the same value for all the series observed, and C may be 

 said to determine the spread of the series, corresponding 

 intervals between the wave-numbers being larger for larger 

 values of C. As B is approximately known two wave- 

 lengths of a series suffice to determine the constants A and 

 C, and thus to calculate approximately the wave-lengths of 

 the other lines. It was by this means that we succeeded in 

 disentangling the spectrum of the gas in cleveite, and 

 showing" its regularity. 



"In the spectrum of many elements two series have been 

 observed for which A has the same value, so that they both 

 approach to the same limit. In all these cases the series 

 for which C has the smaller value, that is to say, which has 

 the smaller spread, is the stronger of the two. In the 

 spectrum of the gas in cleveite we have two instances of 

 the same occurrence. One of the two pairs of series, the 

 one to which the strong yellow double line belongs, consists 

 throughout of double lines whose wave-numbers seem to have 

 the same difference, while the lines of the other pair of series 

 appear to be all single. Lithium is an instance of a pair of 

 series of single lines approaching to the same limit. But 

 there are also many instances of two series of double lines 

 of equal difference of wave-numbers ending at the same 

 place as sodium, potassium, aluminium, etc. There are also 

 cases where the members of each series consist of triplets of 

 the same difference of wave-numbers, as in the spectrum of 

 magnesium, calcium, strontium, zinc, cadmium, mercury. 

 But there is no instance of an element whose spectrum 

 contains two pairs of series ending at the same place. This 



