Irom Newton to Kirchoff. 



211 



coincidence of the sodium lines with the line D of Fraun- 

 hofer, I obtained a tolerably bright solar spectrum and 

 brought a flame colored by sodium vapor in front of the 

 slit. I then saw the dark lines D change into bright ones. 

 The flame of the Bunsen's lamp threw the sodium bright 

 lines upon the solar spectrum." In this experiment the 

 exact coincidence of lines was established. But he pro- 

 ceeds : " In order to find out the extent to which the inten- 

 sity of the solar spectrum might be increased without 

 impairing the distinctness of the sodium lines, I allowed 

 the full sunlight to shine through the sodium flame upon 

 the slit, and to my astonishment I saw that the dark lines 

 D appeared with an extraordinary degree of clearness.' 

 The bright sodium line in this experiment had increased 

 the darkness of the solar line, in other words, the bright 

 line had become a dark one. Here was the key to the 

 mystery that had so long perplexed all observers. Kirchoff 

 had actually made a Fraunhofer line, and in a way which 

 left no doubt of its relation to a bright terrestrial line 

 whose origin was known. His sodium flame had given 

 him bright yellow lines, but the more powerful sunbeam 

 shining through it had changed them into darkness. Kirchoff 

 saw that the sodium flame giving him yellow light had at 

 the same time absorbed the same kind of yellow rays from 

 the sunlight, and had thus cut a slit of color out of the 

 otherwise brilliant spectrum. Eo wonder that the saga- 

 cious mind w T hich so quickly saw this explanation of the 

 phenomenon that had astonished him, should have in- 

 stantly caught the suggestion that the dark lines D of the 

 solar spectrum were themselves originally due to a similar 

 action, — the absorbing action of the glowing vapors in 

 the sun's atmosphere, through which the intenser light of 

 the orb itself was shining. What confirmation more was 

 needed to warrant the startling conclusion, that sodium is 



