A.—MATHEMATICS AND PHYSICS. 45 
with the planes of the orbits at a series of angles other than 120°. On 
this basis he has been able, by taking for granted the dynamical legiti- 
macy of the crossed orbit system, to calculate values for the ionisation 
Parhelium 
Orthohelium 
0 50000 100000 150 000 200 000 =V 
FG. 7.—Scheme of He Lines. 
and other excitation potentials that are in remarkably good agreement 
__with the experimental values found by Frank and Knipping, Horton, 
and others. 
TABLE II. 
Lines Series Calculated HD & K. q 
Observed Repre- | Excitation E ean Remarks 
by Lyman | sentation | Potentials Pote eae 
otentials 
— 1, S—2, s 19-77 19-75 Transition voltage connoting 
change from parhelium to ortho- 
helium. 
600-5 A | 2,S—1,8 20-55 20°55 | A weak radiation and one not pro- 
vided for by the principle of 
selection. 
584-4 ,, 2P—1,8'| . 21°12 21-2 First member of absorption series 
| of parhelium. 
537-1 ,, | 3,P—1,S 22:97. | 22:9 Second member of absorption series 
| of parhelium. 
§22°3 ,, | 4,P—1,S| 23-62 | oo Third member, &c. 
Ripe’, |.0,£—1,9| 23:92 || Fourth member, &c. 
602:0.,, | — —1,S| 245 | 246 Series limit and ionisation poten- | 
(calculated) tial. 
