1879.] Mr. J. N. Lockyer. Spectral Phenomena. 425 



I. "Note on some Spectral Phenomena observed in the Arc 

 produced by a Siemens' Machine." By J. NORMAN 

 Lockyer, F.RS. Received March 3, 1879. 



In continuation of my work on the spectra of metallic vapours pho- 

 tographed when incandescent in the electric arc, I have recently 

 employed a Siemens' dynamo-electric machine driven by a gas engine 

 of ten horse-power, using an effective force of five or six. 



The greatly increased length of arc obtained by these means has 

 enabled me to observe and photograph a new set of phenomena of 

 great beauty, and I think of the highest theoretical importance. 



In my former work with a battery of thirty cells, in order to obtain 

 the lengths of the lines, it was necessary, in consequence of the short- 

 ness of the arc, to throw an image of a horizontal arc on the vertical 

 slit of the spectroscope. In this mariner perfectly symmetrical photo- 

 graphs were obtained, the shortest lines due to the core, and the 

 middle portions of the longest ones proceeding both from the core and 

 the exterior portion, lying in the axis of the photograph. 



With the Siemens' machine the arc is not only much longer, but 

 when some substances are introduced into it, it is accompanied by a 

 flame sometimes three or four inches long, of great complexity both 

 with regard to colour and concentric envelopes. 



The spectrum of this flame was first photographed side by side with 

 that of the arc itself, and when the poles are clean the flame has been 

 shown by eye observation to be chiefly due to the oxidation of the 

 carbon and calcium vapours which exist in the free state in the air, 

 thus giving us absolute demonstration of a combination brought about 

 among vapours by reduction of temperature. 



These flame phenomena also give us an opportunity of observing 

 the inverse appearance of spectral lines, to which my attention has 

 lately been much drawn. The following is a case in point : — In one 

 photograph of the flame given by manganese the line at wave-length 

 4234*5 occurs without the triplet near wave-length 4030, while in 

 another photograph the triplet is present without the line at 4234*5. 



The various phenomena presented in these photographs, especially 

 the greater breadth of the reversals in the case of some of the metallic 

 lines in the flame, and the gradual introduction of new spectra, 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, focussing that particular 

 light which I was about to photograph. 



In the plates thus obtained the spectra of those portions of the arc 

 adjacent to the positive and negative poles are widely dissimilar. 



VOL. XXVIII. 2 T 



