254 Mr. J. R. Capron on the Comparison of some Tube 



Tested for distance. — In the case of the C.A. tube at 18 

 inches from the slit the continuous spectrum and fainter lines 

 disappeared, while the four principal lines still shone out, that 

 in the green being the strongest. At 24 inches the same 

 lines were still visible, though somewhat faintly. 



In the case of coal-gas, at 24 inches the whole spectrum was 

 quite brilliant, the four principal lines being very bright, and 

 even preserving their distinctive colours. The H line, near the 

 line or band in the blue, was also plainly seen. The O tubes, 

 when treated by the same current as the carbon-tubes, were 

 found to be all three identical in general features. The discharge 

 lighted up each of the tubes feebly and somewhat intermittently. 

 Grey in the bulbs, and a faint but decidedly pinkish white in 

 the capillary part, were the distinguishing light colours, while 

 nothing could be more marked than the difference in brilliancy 

 between these and the preceding carbon-tubes. 



The OH 2 tube presented very much the same character; but 

 the discharge occasionally varied from a pinkish white to a yellow 

 colour, somewhat like what artists call brown pink, reminding 

 one of the u golden rays " in certain aurorse. These spectra 

 presented, in common with the carbon-tubes, three principal 

 bright lines or bands in the yellow, green, and blue, with 

 a fainter one in the violet, all shading off towards the violet. The 

 bands, however, showed but very little trace of local colour ; and 

 the whole spectrum had a faint and washed-out look, very 

 different from the carbon-spectra. (I certainly by a little ma- 

 nagement subsequently succeeded in getting the same look to 

 the C.A. spectrum ; but it was only by removing the tube to 

 some distance from the slit, and thus depriving the spectrum of 

 very much of its brightness.) The hydrogen line (solar F) was 

 bright, more so than any of the lines. The intensity of 

 the three principal lines seemed to me to run in the following 

 order : — 



Yellow. Green. Blue. 



Coal-gas /3 a 7 



Oxygen y a /3 



Between the lines 7 and a in the Geissler tube I found a 

 rather bright line, which I shall have occasion to refer to hereafter. 



At 12 inches distance from the slit the spectrum lost 

 nearly all its light; the H line and the three lines 7, a, and /3 

 alone faintly remaining, a being decidedly the brightest. At 24 

 inches no spectrum at all was to be seen. 



I carefully compared together the three principal lines of the 

 two spectra of coal-gas and by means of : — 



1st, the photographed micrometer before described ; 



