252 



SCIENCE PROGRESS. 



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 

 the celestial hieroglyphics we call spectral lines 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 

 orange line was "at D, or near D ". We see from the 



D 1 D 2 



Fig. 5. — The want of coincidence of the orange line D 3 with the dark 



lines D 1 and D 2 . 



diagram (Fig. 5) 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 with any of the dark 

 lines in the ordinary solar spectrum. We were therefore 

 perfectly justified in attaching considerable 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 3 to distinguish it from the sodium lines D 1 and D 2 . 



A considerable amount of work was done with regard 

 to the orange 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 a 

 vacuum tube, and passing an electric current through it, 



