406 >ir Yunnan !."< 1 



The lines in Group I, although appearing in the spark spectrum, are 

 stronger in that of the arc, and therefore cannot he classed as enhanced 

 lines. They are both given by Kowland in his " Table of Solar Wave- 

 lengths "as being coincident with lines in the Fraunhofer spectrum, and 

 may be considered as the lines of silicium which make their appearance 

 at the lowest of the temperatures we are now considering. 



It will be seen that only the stronger line of the two is represented 

 in the spectra of the stars included in the table, and that only in 

 a Cygni, which has been placed lowest in order of ascending tempera- 

 ture among those referred to from a previous investigation of lines in 

 its spectrum other than those of silicium. 



It does not appear to exist at the higher stages of stellar temperature 

 represented by ft, y, or e Orionis. The absence of the other line from 

 the spectrum of a Cygni may be accounted for by its comparative 

 weakness in the silicium arc spectrum. In a Cygni only the very 

 strongest of the arc lines of iron, manganese, &c., are represented, ami 

 then only as very weak lines. 



The lines in Group II are either absent from the most recent arc 

 spectrum photographed at Kensington or exist there only as weak lines. 

 The members of this group are prominent both in the vacuum tube 

 spark and in the spark between poles of silicium, but are upon the 

 whole more prominent in the latter spectrum. Considering the first 

 five lines in the group, which are the only ones comparable with the 

 Kensington records of stellar spectra, a glance at the table will show 

 that they are at their maximum intensity at the stage of temperature 

 represented by a Cygni, and decline in intensity as we pass to the 

 higher successive stages represented by ft, y, and e Orionis. At the 

 latter stage some of them have disappeared, and the others are on the 

 verge of extinction. With regard to the remaining two lines of this 

 group, those at XX 5042 and 5057, the position of which cannot be 

 estimated more accurately than to the nearest tenth-metre on account 

 of the diffuseness of the lines, it is extremely probable that if better 

 photographs of that region of the spectra of a Cygni and Rigel were 

 available, lines would be found corresponding to these silicium lines. 

 Keeler has recorded* a line in the spectnun of Rigel at X 5056, and 

 this is probably identical with the silicium line at X 5057, which is by 

 far the stronger of the pair. 



The lines in Group III occur both in the vacuum tube spectrum of 

 silicium bromide and in the spark spectrum. They appear as a well- 

 marked triplet in the latter, but not nearly so prominently as in the 

 former. 



They first make their appearance in stellar spectra in a Cygni, where, 

 however, they can only just be traced. They are a little stronger in 



* Ast. and Ast. Phjs.,' 1894, vol. 13, p. 489. 



