

xxiv.] A CASE IN POINT. ^51 



expanded, and developed apparently at the expense of 4918, 

 which becomes thin. Taking the jar out, we come back to a 

 result which is very much like the solar spectrum, with the 

 difference, however, that 4918 is somewhat less intense. 



Then come the facts which have already been brought forward 

 (antb page 335) with special reference to these particular lines, 

 that the two which are seen alone in the arc are seen alone 



Fio. 116. Diagram showing the behaviour of three iron lines under different con- 

 ditions, solar and terrestrial. 1, solar spectrum ; 2, arc ; 3, quantity coil with 

 jar ; 4, quantity coil without jar ; 5, intensity coil with jar ; 6, intensity coil 

 without jar; 7, spots observed at Kensington; 8, prominences observed by 

 Tacchini ; 9, prominences observed by Young ; 10, reversed in penumbra of 

 spot observed on August 5, 1872, by Young; 11, motion indicated by change 

 of refrangibility. 



in the spots, or at all events in 73 spots out of 100; and the 

 other line which is so enormously expanded when we use the 

 highest temperature is seen alone in 52 out of 100 prominences 

 by Tacchini. Then we finally learn, that in several cases when 

 a change of refrangibility has been observed in the iron lines 

 in the spots visible on the sun, that the two lines 4918 and 

 491 9'8 have been affected, while 4923'2 has remained at rest. 



