538 SCIENCE PROGRESS 



mentioned in passing that besides the constants a and /3, 

 there are for most of the elements investigated a third set of 

 constants 7 which determine other series for each element than 

 the principal and sharp, series usually called " diffuse " or 

 " nebulous," these being given by the /3 and 7 constants, viz. 



v = (m, /3) — (w, 7) 



m being but equal to 1 or 1-5 and n taking up the successive 

 integral values. Other names in the literature of the sub- 

 ject for the nebulous and sharp series are " first subordinate " 

 and " second subordinate " respectively. Furthermore accom- 

 panying the lines of the series mentioned there have been 

 observed for some elements neighbouring lines called " satel- 

 lites " which are amenable to similar formal expression for 

 their wave-numbers. As stated, the whole matter is involved, 

 and requires careful study in order to avoid pitfalls, and these 

 remarks are only to be taken as a hint of the methods which 

 have been employed to bring order into an apparently un- 

 related mass of phenomena. 



Coming now to Prof. McLennan 's paper, he refers to a 

 series of researches in which the heated vapour of some metals 

 when traversed by electrons with sufficient kinetic energy are 

 stimulated to emit line spectra. Thus, to quote a now classi- 

 cal instance, if heated mercury vapour is traversed by electrons 

 with the kinetic energy acquired in passing between points, 

 from the one to the other of which there is a fall on potential 

 of 4-9 volts, the vapour emits a monochromatic radiation 

 2536-72 A.U. This result is closely related to the quantum 

 hypothesis, for it is easily calculated that 



Ve = hf 



where V is the P.D., 4-9 volts, expressed in electrostatic units, 

 e is the electronic charge, h Planck's constant, and / the fre- 

 quency corresponding to the above wave-length (/ = v.c, 

 v being the wave-number and c the velocity of light). 



McLennan, among others, has carried on researches in similar 

 directions. Thus with zinc vapour the radiation emitted was 

 3075*99 A.U., with cadmium 3260-17 A.U., with magnesium 

 2852-22 A.U. Moreover the impact voltages necessary to 

 stimulate this radiation agreed with the quantum relation 

 written above. 



