and other Spectra with the Spectrum of the Aurora. 255 



2nd, a comparison-prism on the slit plate; 



3rd, a piece of very fine brass foil cut as a pointer and 

 fixed in the focus of a positive eyepiece. 



The lines or bands in both tubes were found to be slightly- 

 nebulous towards the less-refrangible end (where they were mea- 

 sured), and the tube was not bright under a moderately high 

 power (positive eyepiece); but, subject to these remarks, the three 

 principal lines in both tubes were found to correspond in position 

 within the limits of my instrument. The spectra did not, how- 

 ever, I am bound to say, look alike. 



I was naturally puzzled by these observations ; and it then 

 occurred to me to reduce Dr. VogePs spectrum of 0, given in 

 his memoir, to the same scale with my own. This I did inde- 

 pendently ; and I then compared the result with my own spectrum 

 as mapped out ; and from it I judge that if my tubes, one and 

 all, showed a carbon-spectrum, the learned Doctor's tube must 

 have been subject to a similar infirmity, as the tubes all agree 

 in main features. 



There is, however, one point to which I desire to draw atten- 

 tion, which is this, that common to both the Doctor's and my 

 own Geissler spectrum I find the before-mentioned rather bright 

 line between y and a. This line I find no equivalent for in either 

 of the carbon-tubes. For spectra of coal-gas and oxygen see 

 Plate IV. spectra 2, 3 & 4. 



In comparing the spectra, the reader should remember that the 

 tube- and flame-spectra of carbon do not correspond. Compare, 

 for instance, the spectrum of coal-gas or C.A. in tube and the 

 well-known lines or bands in the blue base of a candle-flame. 

 The sharper edge of the yellow line or band of the carbon-tubes 

 will be found about midway between the two brightyellow candle- 

 lines or bands. The first of the very beautiful group of lines or 

 bands in the green in the candle-flame falls considerably behind 

 the sharper edge of the green line or band in the tube, while the 

 third bright band in the tube alone of the three corresponds with 

 a very faint band in the candle-flame, and a line or band in the 

 violet in the tube-spectrum finds no equivalent in the candle- 

 spectrum. For comparison of the carbon -tube and flame-spectra 

 (the principal lines of the tube being alone given) see Plate VI. 

 spectra 6 & 7. 



From Dr. Watts's ' Index of Spectra ' I have extracted the 

 three principal carbon-tube bands or lines ; and they compare 

 with Dr. VogeFs oxygen-tube as under : — 



Yellow. Green. Blue. 



Dr. VogePs oxygen-lines 5603 5195 4834 

 Dr.Watts's carbon- tube ) 



bands or lines . / 



622 5189 4829 



