xvi.] REVERSAL PHENOMENA. 223 



Very often on the opposite pole the line is seen merely as a 

 bright one, or again the absorption is reduced to its smallest 

 proportions. 1 



The following general statements may be made with regard to 

 these curious phenomena : 



I. We have first a general absorption of the light of the arc 

 over the region to be eventually occupied by the bright line. 



II. Next the disappearance cf this indefinite absorption and 

 the formation of a truncated absorption of a symmetrical bright 

 and wider line. 



III. Next the parallelism of the boundaries of the bright and 

 dark lines in the centre of the arc itself. 



IV. Next the various absorption phenomena on the two poles. 



V. Finally the extinction of the absorption line in the arc. 

 Such phenomena afford a striking instance of the irregular 



absorption and radiation of the 'molecules of the same element 

 in the same sectional plane of the arc. 



Some lines are clean cut in their reversal ; others, again, to 

 use the laboratory phrase, are " fluffy " to a degree that must be 

 seen to be appreciated, so much so, that when photographed 

 they appear merely as blurs upon the plate. 2 In some cases 

 the reversal is seen to widen as we approach the cooler 

 external region of the arc, thus showing absorption increasing 

 with reduction of temperature. 



When a Siemens' lamp is employed, the absorption phenomena 

 of the flame are also most curious. The lines which reverse 

 themselves most readily in the arc are generally those, the 

 absorption of which is most developed in the flame ; thus the 

 manganese triplet in the violet is magnificently reversed in the 

 flame, and the blue calcium line is thus often seen widened, 

 H and K "being not only not absorbed, but entirely invisible. 



1 Proc. Roy. Soc. March, No. 194, 1879. 



2 Proc. Roy. Soc. 194, 1879. The counterparts of these lines are almost 

 invariably absent from the solar spectrum. 



