320 



NATURE 



[February 6, 1896 



of the sun, formed by a telescope, on to the slit of a 

 .spectroscope, so that the spectrum of the sun's edge and 

 of the sun's surroundings can be seen at the same time, 

 exact coincidence or want of coincidence between the 

 bright and dark lines can be at once determined. I may 



B c 



X) 



^f 



Fig. I. — Pogson's diagram of the spectra of the sun's surroundings in the Eclipse of i8i 

 The bright lines seen are shown in the upper part of the diagram ; the chief lines 

 the solar spectrum, red to the left, blue to the right, are shown in the lower part. 



remind you that during an eclipse this is not possible, 

 as the ordinary spectrum of the sun, with its tell-tale 

 dark lines, is invisible because the sun, as we ordinarily 

 see it, is hidden by the moon. 



F G H 



aBC 



Fig. 2. — Summation of the observations of the spectrum of the sun's 

 surroundings in the Kclipse of 1868. (i) Solar spectrum showing the 

 position of the chief lines (2) Rayet's observations of bright lines. 

 (3) Herschel's observations of bright lines. (4) Tennant's observ.itions. 



Working, then, under such very favourable conditions, 

 it was seen that there was certainly a red line given by 

 this lower part of the solar atmosphere coincident with the 

 very important line in the solar spectrum which we call C. 





Fig. 3. 



-The exact coincidence of the red line with the dark line C 

 determined by the new method. 



Another part of the spectrum in the blue-green was 

 examined, and there again it was seen that the parts 

 outside the sun gave us a bright line exactly in the 

 position of the obvious dark line in the solar spectrum 



NO. 1 37 I, VOL. 53] 



which is called F ; so that with regard to those two most 

 important lines, there was no doubt whatever that we 

 were dealing with the substance which produces these 

 dark lines in the solar spectrum. 



Fig. 5 is a diagram of the yellow, or rather the orange, 

 part of the solar spectrum, showing two very 

 important lines, which are called the lines D, 

 due to the metal sodium, the investigation of 

 which was just as important in solving these 

 celestial hieroglyphics as the Rosetta stone 

 was important in settling the question of the 

 Egyptian ones. 



Pogson, in referring to the eclipse of 1868, 

 said that the yellow line was "at U, or near 

 D." You will see from this diagram that the 

 new method indicated that " near D " was the 

 true definition. The line in this position in 

 the spectrum, unlike the other two lines which 

 I have indicated, has no connection at all 

 I with any of the dark lines in the ordinary solar spectrum. 

 We were therefore perfectly justified in attaching con- 

 siderable importance to this divergence in the behaviour 

 of this line, taking the normal behaviour to be represented 

 by the two strong lines in the red and the blue-green. 

 The new line was called D'' to distinguish it ixom_ the 

 sodium lines D^ and D-. 



Fig. 4. — The exact coincidence of the blue-green line with the dark line F. 



A considerable amount of work was done with regard 

 to the yellow line. It was found that there was no sub- 

 stance in our laboratories which could produce it for us, 

 whereas in the case of the line D we simply had to burn 

 some sodium, or even common salt, in a flame to produce 

 it, and the other lines in the red and the blue-green 

 were easily made manifest by just enclosing hydrogen in 



; of the range line, D^, 

 Dl and D2. 



th the dark lines 



a vacuum tube, and passing an electric current through 

 it, or observing the spectrum of a spark in a stream of 

 coal-gas. 



Now at the first blush it looked very much as if this 



