142 THE CHEMISTRY OF THE SUN. [CHAP. 



electric arc or spark in which the pure vapour of the substance 

 which is being rendered incandescent fills the whole interval 

 between the poles, the number of particles being smaller and 

 the degree of incandescence being less intense at the sides of 

 the arc. 



Take, for instance, the case of the volatilisation of iron in an 

 electric arc. Since light from every part of the arc placed in 

 front of a slit must enter every part of it, the differences 

 between the light proceeding from the upper pole or the lower 

 pole, or from the globule of iron which is being melted and 

 exists in a liquid form, or from the vapours of iron which 

 surround that liquid globule if there are any such differences 

 are absolutely lost in observations made in the ordinary 

 way. But if we introduce the lens between the light- 

 source and the slit of the spectroscope ; if, as we throw 

 an image of the sun on the slit, we throw an image of the 

 light-source on the slit, we ought to see any difference that 

 may exist. 



We readily do see that there are very considerable optical 

 differences in the various parts of the image of the light-source. 

 We have the upper and lower pole, the globule of iron vola- 

 tilising, and the vapour, both in the arc, properly so-called, and 

 the accompanying flame, each with its own special spectrum. 



By an easily understood artifice we can throw an image of a 

 horizontal arc on a vertical slit : the slit will give then the spec- 

 trum of a section of the arc at right angles to its length. The 

 vapour which exists furthest from the core of the arc has a 

 much more simple spectrum than that of the core of the arc 

 itself, the spectrum of the core consisting of a large number of 

 lines, all of which die out until the part of it furthest from the 

 centre gives but one line. 



It is obvious that if we throw the image of the electric arc 

 on the slit in this manner we can examine the vapour without 

 being inconvenienced by the bright continuous spectrum of 



