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bright solar spectrum and brought a flame colored by sodi- 

 um vapor in front of the slit. I then saw the dark lines D 

 change into bright ones. The flame of a Bunsen's lamp 

 threw the bright sodium lines upon the solar spectrum with 

 unexpected brilliancy. In order to find out the extent 

 to which the intensity of the solar spectrum could be in- 

 creased without impairing the distinctness of the sodium 

 lines I allowed the full sunlight to shine through the sodium 

 flame and to my astonishment, I saw that the dark lines D 

 appeared with an extraordinary degree of clearness. I 

 then exchanged the sunlight for the Drummond's or oxy- 

 hydrogen lime light, which, like that of all incandescent, 

 solid or liquid bodies, gives a spectrum containing no dark 

 lines. When this light was allowed to fall through a suita- 

 ble flame, colored by common salt, dark lines were seen in 

 the spectrum in the position of the sodium lines. The 

 same phenomenon was observed if instead of the incan- 

 descent line a platinum wire was used, which being heated 

 in a flame was brought to a temperature near its melting 

 point by passing an electric current through it. The phe- 

 nomenon in question is easily explained upon the suppo- 

 sition that the sodium flame absorbs rays of the same 

 degree of refrangibility as those it emits, whilst it is per- 

 fectly transparent for all other rays. 



This opacity of heated sodium vapor for the particular 

 kind of light which it is capable of giving off, was strikingly 

 exhibited by Prof. Eoscoe in one of a course of lectures on 

 spectrum analysis lately delivered by him in London, at 

 the Royal Institution- 



A glass tube containing a small quantity of metallic sodi- 

 um, was rendered vacuous and then closed ; on heating the 

 tube, the sodium rose in vapor, filling a portion of the 

 empty space. Viewed by ordinary white light, this sodium 

 vapor appeared perfectly colorless, but when seen by the 

 yellow light of a soda flame the vapor cast a deep shadow 

 on a white screen, showing that it did not allow the yellow 

 rays to pass through. 



