554 Production of a Spectrum by Anomalous Dispersion. 



the sodium vapour is very dense, we see only a single bright 

 line bordering D 2 , owing to the complete absorption of the 

 light between the lines. 



I next instituted a search for the light of a wave-length 

 slightly greater than that of the sodium lines. For these 

 waves the vapour has a refractive index greater than 1, conse- 

 quently the rays will be concave downward in the layer of 

 vapour. (The paths are indicated on an exaggerated scale in 

 fig. 1.) If we move our prismatic telescope down in a search 

 for these rays, the solar spectrum will appear and drown out 

 everything, but if we set up a screen (shown in fig. 3) in 



Fig. 3. 



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I METAL PlAnj ~ - - Z--1*\-S " % 



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such a position as just to cut off the light from the illuminated 

 spot, and feed the flame with sodium, we shall presently see 

 bright lines appear on the side of the sodium lines towards 

 the red. In this case, when the vapour is dense we get only 

 a single line bordering D Jm 



The arrangement described is inconvenient in many ways 

 to work with, and I accordingly modified it in the following 

 way : — 



The light of an arc-lamp (fig 3) is focussed on a horizontal slit, 

 and a flat metal plate supported so that the plane in which 

 its under surface lies coincides with the plane of the slit. 

 The plate should be an inch or so thick, with a fairly level 

 surface. At a distance of about two metres a telescope pro- 

 vided with a prism (direct-vision if possible), arranged so as to 

 give a vertical spectrum, is placed at such a height that the 

 prism barely catches the rays coming from the slit and 

 grazing the surface of the plate. On looking into the 

 telescope we see a bright continuous spectrum, and the tele- 

 scope is to be raised until this becomes quite faint. The 

 Bnnsen-burner beneath the plate is now lighted, and a bit of 

 sodium in a small iron capsule introduced into the centre of 

 the flame. The results obtained are practically identical with 

 those which have been described. I am now arranging 

 apparatus by which 1 hope to obtain similar flash-spectra by 

 the dispersion of vapours exhibiting more complicated absorp- 

 tion spectra than sodium. If these experiments are successful, 

 much can be learned by comparing the flash-spectra with the 

 emission-spectra. If it be found that certain lines are absent 



