XL] LOCALIZATION OF PHENOMENA. 143 



the light from either of the carbon poles. It is also obvious 

 that if we arrange the slit horizontally while the current is 

 passing in a vertical direction from one pole to the other, 

 we shall be able, by moving the slit upwards, to see if there are 

 any differences observable in the vapour, first in the region 

 where we have intense boiling and volatilisation going on, and 

 in the necessarily cooler region where the arc is in contact with 

 the outer air. 



In this way the spectrum of each substance furnishes us with 

 long and short lines, the long lines being common to the more 

 and the less intensely heated parts of the arc, because we are 



FIG. 61. Arrangement for obtaining long and short lines. Image of the 

 horizontal arc on slit-plate of spectroscope. 



not dealing with a simple section, and the short lines being 

 confined to the more heated ones only. 



Whether we use the artifice of a horizontal arc with a vertical 

 slit, or a vertical arc with a horizontal slit, does not matter, pro- 

 vided we keep the slit immersed in the light of the arc, and thus 

 get rid of the light from the poles, and at the same time arrange 

 the slit so that we can compare the light in the interior portion 

 of the light-source with the light nearer its boundaries if we 

 take all these precautions we shall then get in the case of every 

 substance such a result as shown in Fig. 62. We have in the 

 centre a complete spectrum, its intensity being gradually toned 



