Atomic 7 heavy and Low Voltage Ave* in Cesium Vapour. 81 



gained by one unfamiliar with series notation from an exami- 

 nation of the series formula?. The first doublet of the 

 principal series may be represented by electrons falling from 

 the 2p Y and 2/) 2 rings into the 1*55 ring, thus emitting the 

 lines I 'ids — 2p±. The second doublet of this series is repre- 

 sented by electrons falling from the 3jt?i and 3/) 2 rings into 

 the X'Ds ring, giving rise to l"5s — 3pf, and so on. The lines 

 of the 1st subordinate series are represented by electrons 

 falling from the md rings into the 2p 1 and 2p 2 rings ; 

 the lines of the 2nd subordinate series by electrons fall- 

 ing from the ms rings into the 2p { and 2p 2 rings ; the 

 lines of the Bergmann series by electrons falling from the 

 m&p rings into the d rings. Combination lines are repre- 

 sented in the same manner. Thus the line 2*5.9 — 3jt?i arises 

 from an electron falling from the 2>p x ring into the 2'bs ring. 

 Few of the possible combination lines are probable enough 

 to appear in spectroscopic measurements. For example, the 

 line 2pi—4:pi, while possible, represents an extremely impro- 

 bable type of transition, and has never been observed in the 

 caesium spectrum although it is known for sodium. There 

 are other ways much more likely for an electron to leave 

 the 4/?i ring. 



On the basis of the Bohr theory, the lines shown in the 

 diagram are really portions of the elliptical or ring orbits 

 about the nucleus, which is located off the figure in the right. 

 Surrounding the nucleus are the X-ray rings and many other 

 orbits between the nucleus and the l'ds ring in which electrons 

 may be present. 



In the unexcited caesium atom no electrons exist outside 

 of the 1*05 ring. This ring represents the outermost stable 

 orbit and the innermost unstable orbit of the normal atom, 

 and the diameter of this orbit should give the diameter of 

 the normal caesium atom. The evidence for this is fairly 

 conclusive. That there are electrons in this ring is shown 

 by the existence of the principal series, which converges at 

 1*5*. That it represents the innermost unstable orbit for 

 any ordinary method of excitation is shown by the fact that 

 no spectroscopic series of caesium converges at a higher fre- 

 quency than 1*05, nor is any line of higher frequency known. 

 That it represents the outermost stable orbit is evidenced by 

 determination of the ionization potential for which Foote, 

 Rognley, and Mohler* observed the value 3"9 volts. On the 

 basis of the quantum relation, Iw — eV, an electron falling 

 through 3"877 volts possesses just sufficient energy to eject 



* riiys. Rev. xiii. p. 59 (1919). 

 Phil. Mag. S. 6. Vol. 40. No. 235. July 1920. G 



