CHAP, ix.] THE PRINCIPLE INVOLVED. 107 



that by employing a considerable number of prisms we ought to 

 be able to abolish the illumination of our air altogether, and in 

 that way we should no longer be limited to determining merely 

 the chemical nature of the spots, we should be equally able to 

 determine the nature of the surrounding solar atmosphere, 

 supposing the phenomena observed during eclipses were really 

 due to incandescent vapours at the sun, and were not lunar or 

 terrestrial. 



This principle can be very clearly demonstrated by means of 

 the electric light. The lower pole is charged with some metallic 

 salt, say a salt of lithium, and a single prism is interposed in the 

 beam. There then appears on the screen a mixed spectrum, due 

 partly to the continuous light from the solid poles, and partly 

 to the lithium vapour. The red lithium line stands out on a 

 background v of continuous spectrum. On mounting a second 

 prism, the continuous spectrum from the poles being much more 

 dispersed is enfeebled, while the bright line of lithium retains 

 almost its original brilliancy because it is not dispersed. On 

 the addition of a third prism this effect is enhanced. 'In fact, 

 the brightness of the line relatively to that of the continuous 

 spectrum increases so rapidly with each increase of dispersion, 

 that by employing a sufficient number of prisms, we can practi- 

 cally abolish the latter and see the bright line on a dark ground. 

 That was the principle which it was suggested would enable the 

 spectroscope to be used in making what have been called 

 artificial eclipses.' 1 



This method was first applied by Janssen immediately after 

 the eclipse of 1868. 



Now if we consider what are the conditions presented by 

 eclipses, as well as the sort of thing the spectroscope is called 

 upon to observe, we shall see the very considerable advantage of 

 the introduction of the new method. In the first place, eclipses, 

 which are so full of teachings to be got only at the moment 



1 Proc. Roij. Soc. 1866, vol. xv. p. 258. 



