The OH₂ tube presented very much the same character; but the discharge occasionally varied from a pinkish white to a yellow colour, somewhat like that which artists call brown-pink, and reminding one of the “golden rays” in certain Auroræ. These O 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 management, 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 O lines. The intensity of the three principal lines seemed to me to run in the following order:— Yellow. Green. Blue. Coal-gas β α γ Oxygen γ α β Between the lines γ and α in the Geissler O tube I found a rather bright line, which I shall have occasion to refer to hereafter. At 12 inches distance from the slit the O spectrum lost nearly all its light; the H line, and the three lines γ, α, and β, alone faintly remaining, α being decidedly the brightest. At 24 inches no spectrum at all was to be seen. [Sidenote: Comparison of spectra of coal-gas and O tubes.] I carefully compared together the three principal lines of the two spectra of coal-gas and O by means of:— 1st, the photographed micrometer before described; 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 measured), and the O tube was not bright under a moderately high power (positive eyepiece). 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, however, I am bound to say, _look_ alike. [Sidenote: Dr. Vogel’s O spectrum reduced and compared.] Puzzled by these observations, it then occurred to me to reduce Dr. Vogel’s spectrum of O, given in his memoir, to the same scale with my own. This I did independently, and I then compared the result with my own spectrum as mapped out. From the comparison, I judge that if my O 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 agreed in main features.