and other Spectra with the Spectrum of the Aurora. 253 



effect of distance upon the spectrum. The slit was made rather fine. 

 At 6 inches distance from it the line a (see drawing) in the blue- 

 green (F solar) was very bright. The lines marked /3, y, 8, e, 

 and f also survived, but were faint. At 12 inches from the slit 

 a and 7 were alone seen ; and at 24 inches a stood by itself upon 

 a dark ground. I also noticed that the red and yellow parts 

 of the spectrum first lost their light on the tube being withdrawn 

 from the slit ; and this appeared to account for ft disappearing 

 while 7 survived. For drawing of the hydrogen-spectrum see 

 Plate IV. spectrum 1. 



Carbon and Oxygen Tubes. 



By way of introduction let me state that I do not here pro- 

 pose to enter into the question now vexed by Professor Piazzi 

 Smyth as to the real character of the carbon-spectrum, and that 

 when I speak of carbon-tubes and spectra I use the words in a 

 general sense, and as meaning those tubes and spectra which 

 have hitherto passed under that designation. 



The following tube-observations have been taken together, 

 because my friend, Mr. Henry R. Procter (to whom I am in debt 

 for many profitable hints and suggestions in auroral work) is dis- 

 posed to regard the spectra found in the carbon-tubes, and in 

 those marked " O" as identical; considering that pure 

 with the ordinary non-intensified discharge gives only a con- 

 tinuous spectrum, and that the tubes are in fact generally 

 lighted up by a carbon-spectrum, the result of impurity from 

 accidental causes. The tubes examined for the purpose of com- 

 parison were as follows :— A coal-gas tube, a tube marked 

 " C.A.," three tubes, two of (I believe) London make, and 

 the third from Geissler, and an OH 2 tube, also from Geissler. 

 The carbon-tubes were both brilliantly and steadily lighted by 

 the current. The C.A. tube glowed with a peculiar silvery 

 grey green light in the capillary part, and with a grey glow con- 

 siderably stratified in the bulbs. The coal-gas-tube discharge 

 was whiter and still more brilliant than the C.A., and with 

 even finer stratification in the bulbs. The spectra of both tubes 

 were conspicuous for the same three well-known principal bright 

 lines or bands in the yellow, green, and blue (with one fainter 

 in the violet), all shading off* towards the violet, and in both cases 

 with fainter intervening bands or lines ; and these last bands or 

 lines only partially coincided when the two tubes were com- 

 pared. 



The spectra in both cases were rich and glowing, with a cer- 

 tain amount of continuous spectrum between the lines ; and the 

 three principal bands or lines in both cases showed well and 

 distinctly their respective place-colours. 



