XI.] 



LONG AND SHORT LINES. 



145 



If we take horizons from the central portion of the diagram 

 to the point most distant from that central axis, we find, in 

 the case of every substance, the light at last becomes prac- 

 tically monochromatic. The vapour at this distance from the 

 central axis radiates to us light spectroscopically represented 

 by one line. As we get nearer and nearer the centre 

 of agitation the spectrum becomes more complex, until at 

 length, when very near the central axis, we get a great many 

 short lines introduced, so that the spectrum at that point is 

 most complex. This I am anxious to draw attention to with 



FIG. 63. Spectrum of sodium, showing the long and short lines. 



some insistence, because we shall understand at once the terms 

 long and short lines, about which there will be a great deal in 

 the sequel. 



Figure 63 shows the much more simple spectrum of sodium. 

 The longest line in the middle is D, that to the left, the line 

 in the green, and we find that one set of the double lines 

 excels all the others, and reaches a greater distance from the 

 central axis. 



An electric lamp can be arranged to show the long and short 



L 



