May 20, 1920] 



NATURE 



361 



seconds. This event afforded astronomers an 

 opportunity of applying- the spectroscope, in con- 

 junction with the telescoge, to determine what 

 the prominences were really made of. On this 

 occasion not only were all the expeditions suc- 

 cessful, but an almost identical discovery was 

 iilso made by the numerous observers. 



It was observed that the prominences gave 

 spectra of bright lines, and, with the means of 

 recognition available at the few moments of 

 totality, the red, green, and blue lines which were 

 seen were attributed to the gas hydrogen, while 

 the strong, bright yellow line was stated to be 

 due to the luminous emission of sodium. 



During this eclipse the distinguished French 

 astronomer, Janssen, was so struck with the bril- 

 liancy of the prominence lines in his spectroscope 

 that he considered it certain he would be able to 

 see the bright lines without an eclipse at all. This 



It is interesting as a matter of history to refer 

 here to the first communication which Lockyer 

 made to the Royal Society with reference to his 

 first successful observation. 



October 20, 1868. 



Sir, — I beg to anticipate a more detailed com- 

 munication by informing you that, after a number of 

 failures, which made the attempt seem hopeless, 1 

 have this morning perfectly succeeded in obtaining 

 and observing part of the . spectrum of a solar 

 prominence. 



As a result I have established the existence of three 

 bright lines in the following positions : — 

 (i) Absolutely coincident with C. 



(yiS Nearly coincident with F. 



(iii) Near D. 



The third line (the one near D) is more refrangible 

 than the more refrangible of the two darkest lines by 

 eight or nine degrees of Kirchhoff's scale. I cannot 

 speak with exactness, as this part of the spectrum 

 requires re-mapping. ... 



Fu;. I. — ^[edal struck by the French Government in hon )ur of the joint discovery of the composition of the prominences by Janssen and 



Lockyer in the year i8fc8. 



he did during^ the following seventeen days which 

 he spent at the eclipse station, observing the 

 prominences on the limb of the sun. 



The achievement of Janssen was based upon 

 principles which in 1866 had been placed before 

 the scientific world by Sir Norman Lockyer. 

 Owing, however, to regrettable delays in the 

 delivery of the instrument which was ordered in 

 the beginning- of the year 1867, and being specially 

 made for him from funds supplied from the 

 Government Grant Committee, Lockyer did not 

 receive it until October 16, 1868. He first used it 

 on October 20, observing- the bright lines which 

 had been recorded in the August eclipse. 



Both Janssen and Lockyer communicated the 

 results of their diw:overies to the Paris Academy 

 of Sciences, and these despatches arrived a few 

 minutes of each other on the same day. In honour 

 of the joint discovery the French Government 

 struck a special medal (Fig. i). 

 NO. 2638, VOL. 105] 



From the above it will be noticed that Lockyer 

 gives the position of the bright yellow line as 

 near D, and not coincident with D, D being- the 

 lines of emission of sodium previously referred to. 



With regard to the behaviour of this line, he 

 states in a later communication (November 19, 

 1868) :— 



There is a line in the yellow, most probably pro- 

 ceeding from the substance which gives off the light 

 at C and F, as the length of this line, as far as the 

 later observations with the more correctly adjusted 

 instrument go, is the same as that of those in C 

 and F. 



This statement shows that the yellow line 

 behaved like the lines of hydrogen, and the view 

 put forward then was that probably this line 

 might be due to hydrogen also. The line was 

 called D3 to differentiate it from the double line 

 of sodium Dj and D.,. 



A considerable amount of work was now done 



