THE SPECTRUM 311 



be used. In front of the slit E through which parallel rays 

 are sent from the condensers, is a Bunsen burner, G, in the 

 flame of which is held an iron or platinum spoon I containing 

 a good pellet of sodium, which should burst into vivid com- 

 bustion with some white vapour, In front of the sodium is 

 arranged a screen s with an aperture, to shade off all direct 

 sodium light from the screen hence this experiment is not 

 well adapted for a direct prism. The light from the slit has 

 all to pass through the sodium flame before it reaches the 



FIG. 171. Reversed Sodium Line 



lens, L, and a dark line D appears in the orange-yellow of the 

 spectrum. 



By modifying the arrangement as in fig. 171 (after Miiller), 

 detaching the slit from the front of the lantern and placing 

 the burner B on the opposite side of it from the focussing 

 lens, the lens L and prism p (really two prisms) being as 

 usual, both spectra can be shown together. For introducing 

 by hand another tin screen T between the Bunsen-flame and 

 the condensers, the radiating arc light is cut off from the 

 upper part of the slit, and consequently half the spectrum on 



