In a communication to ‘Nature,’ Mr. H. R. Procter has pointed out an apparent coincidence in position of several of the Auroral lines with those of a spectrum occasionally obtained from air at low pressure with a feeble discharge. It is, he says, sometimes exhibited in lumière (phosphorescent?) tubes, and he believed it, in part at least, to be the spectrum described by Wüllner (Philosophical Magazine, June 1869) as a new spectrum of oxygen. [Sidenote: How obtained.] He had obtained it very vividly in pure electrolyzed oxygen with a feeble discharge, but some perplexing observations made him doubtful of its origin. Plate XI. fig. 4 gives a representation of this spectrum as shown by Mr. Procter, except that my drawing is in black for white. [Sidenote: Compared spectra described.] The upper spectrum is that above mentioned, the centre one that of the Aurora, the lower one the lines of Na and H for comparison. The Auroral yellow-green line, in January 1870, was found by Mr. Procter coincident with a bright line or band in the tube (with a spectroscope of a 60° bisulphide prism, and magnifying-power about six). The third and fifth lines in the Aurora seemed also to correspond with tube-lines. As to these Mr. Procter says they were not bright enough to be compared with the same accuracy as the yellow-green line, but that the positions could not be far wrong. [Sidenote: Mr. Procter’s subsequent views. Yellow-green line traced to some form of hydrocarbon.] Mr. Procter subsequently (‘Edinburgh Encyclopædia,’ art. “Aurora”) considered he traced the yellow-green tube-line to some form of hydrocarbon. On examination with instruments of greater dispersion, it was found that, though more refrangible than the first band of citron acetylene (candle-flame), it was less so than the Aurora-line. The tube-band, too, was shaded towards the violet, which was not the case with the Aurora-line. The question as between hydrocarbon and oxygen I did not then consider as disposed of. With the lumière tubes the question might be open, but I did not see how it could be in the case of the electrolyzed oxygen-spectrum. From a comparison of the tube-spectra, I have shown that although the spectra of the carbon and oxygen tubes are proved to be, photographically, as a whole, distinct, they have, as to position of some of the principal lines in the central part of the spectrum, a very close resemblance. [Sidenote: Probability that O may play a part in the Aurora-spectrum.] That oxygen may in some form play a part in the Aurora seems highly probable; how far it is spectroscopically detected seems a different question. [Sidenote: Difference between air-spark and tube-spectra.]