

xxvii.j BRIGHT LINES IN THE ARC. 383 



region close to the arc ; the coarser ones are born, and live for a 

 time, in a region further removed. 



The lines seen reversed in the flame of a substance incan- 

 descent in the naked arc, are, so far as I know in the case of 

 metals of the iron group, not observed in any solar phenomena, 

 although their record is to be found among the Fraunhofer lines. 

 This is also explained by assuming that there as here, they do 

 not exist at all in a highly heated region. 



Now, if this molecular dissociation goes on at the temperature 

 of the arc, it is worth while to inquire whether there are other 

 appearances put on when the arc is minutely studied which 

 favour this view. 



I now proceed to discuss work of this kind, which, so 

 far as I can see, is entirely in harmony with the above 

 explanation. 



2. Evidences of Separation Exhibited by Bright Lines in 

 the Naked Arc, 



The phenomena presented in the photographs produced by a 

 Siemens' machine are by no means limited to the absorption 

 phenomena which we have just considered. In the case of 

 many substances new spectra altogether made their appearance 

 towards the end of the flame where the lines die out. 



This gradual introduction of new spectra, from core to end 

 of flame, led me to imagine that in different regions of the 

 arc itself the spectroscopic effects might vary greatly ; and to 

 test this I very carefully projected the image of a vertical arc on 

 the slit, focusinsf that particular light which I was about to 

 observe or photograph. 



The greatly increased length of arc obtained by the Siemens' 

 machine has enabled me to observe and photograph a new set 

 of phenomena of great beauty, and I think of the highest 

 theoretical importance. Before I. proceed to discuss them it 

 will be well to study still more deeply the conditions actually 



