1879.] 



attending the Reversal of Lines. 



429 



terrestrial substances, there is no difficulty in seeing the reversal of 

 winged lines in the case of all spectra in which they exist, and such 

 lines lying in the region between K and G have been photographed 

 and exhibited to the Society on several occasions in connexion with 

 one part or another of my researches. When a lamp o£ thirty cells, 

 however, is used, although the various curious phenomena which these 

 reversals present are easily visible, it is very difficult to photograph 

 them. 



The longer arc given us by the Siemens' machine to which I have 

 referred in another communication has enabled me, however, to photo- 

 graph several of the various aspects put on during the process of 

 reversal ; these photographs I exhibit to the Society ; chief among 

 these phenomena are the various thicknesses of the lines of reversal 

 over the arc and poles, and the appearance of the bright line without 

 reversal in some regions, and the reversal without bright line in 

 others. 



All the phenomena presented by the absorption of the D line to the 

 eye are here in duplicate. It may be useful, perhaps, to state what 

 phenomena are seen in the case of the D line, when a small image of 

 the arc, carefully focussed for the yellow light, is thrown upon the 

 slit and considerable dispersion is employed. 



If the arc is observed before the introduction of the sodium on to 

 the poles, with the poles slightly separated, the continuous spectrum 

 of each pole will be bounded by a sharp line, and in the included 

 region the exquisite flutings of the carbon vapour will be seen together 

 with the lines due to any metallic substances . present. The metallic 

 lines will be thickest near one pole, and will overlap its continuous 

 spectrum, while the carbon flutings will overlap the other. The T) 

 lines in the arc should occupy the centre of the field of view. 



If now a piece of metallic sodium be placed on the lower pole, the 

 whole of the light will be blotted out, if the field of view be small. 

 Gradually the two ends of the spectrum of the arc will begin to 

 appear on either side of the field, the sharp boundary lines to which 

 reference has been made having disappeared, as the poles are no longer 

 incandescent. 



The absorption in its retreat to the central region will next take 

 the appearance of a truncated cone, its base resting on that side of the 

 arc formerly occupied by the carbon flutings. The intense blackness 

 gradually changes into a misty veil through which, as it were, the D 

 lines gradually make their appearance as enormous truncated cones 

 with their bases turned in the opposite direction to that occupied by 

 the original absorption. 



The more refrangible line is twice as thick as the other, and is often 

 contorted while the other is rigid. Gradually, as the quantity of 

 sodium vapour is reduced, the poles regain their original incandescence, 



