4 Mr. E. F. J. Love on a Method of Discriminating 



lished a map, extending from F to b, showing among others 

 the arc-spectrum of cerium, and the spectra of the lines seen 

 widened in two sun-spots at Greenwich in 1881. The spec- 

 trum of cerium exhibits numerous coincidences with widened 

 lines, though comparatively few with lines given in Angstrom's 

 map. On the other hand, Messrs. Hutchins and Holden*, 

 who have re-examined the evidence for the existence of certain 

 substances in the sun, with the aid of photography, remark, 

 "So numerous are the lines [of cerium, molybdenum, ura- 

 nium, and vanadium] that often on the photographs the total 

 space occupied by them is greater than the space not so 

 occupied. . . Evidently coincidences between these and solar 

 lines cannot fail to occur as matters of chance and therefore 

 prove nothing. One can easily count a hundred or so such 

 coincidences without the slightest conviction that the con- 

 nexion is other than fortuitous." 



As a complete map of the arc-spectrum of cerium has not 

 yet been published, the writer has been obliged to fall back on 

 the evidence given by Liveing and Dewar's map, mentioned 

 above. It is briefly as follows : — Of the 34 lines included in 

 the map 20 coincide with solar lines which are not already 

 assigned or possibly assignable to other metals; of these solar 

 lines 6 only are represented on Angstrom's map, and the whole 

 have been observed to be widened, most of them very con- 

 siderably, in sun-spots, though not all in the same spot. This 

 fact by itself considerably strengthens the case for the 

 existence of cerium as a constituent of the sun, since it 

 demonstrates a connexion between these lines. Let us apply 

 the method to these twenty coincidences. The differences 

 between the wave-lengths of the cerium and solar lines are 

 given in the following table : — 



Between 0*0 and 0-1 Xth metre, 



;? 



o-i 



)j 



0-2 



jj 



0-2 



jj 



0-3 



jj 



0-3 



jj 



0-4 



V 



0-4 



)) 



0-5 



JJ 



0-5 



» 



0-6 



J5 



0-6 



)} 



0-7 



12 coincidences. 



2 



V 



2 



V 



1 



V 







J J 



2 



■)•) 



1 



J) 



These numbers when plotted give the curve represented in 

 fig. 6. This curve closely resembles rigs. 1, 2, and 3, 

 except in respect of its greater steepness; the latter, so far 

 from being an objection, testifies to the fewness of the 



* Proc. Amer. Acad. Arts and Sci. xxiii.; Phil. Mag. [5] xxiv. p. 826, 



1887. 



