There is, however, one point to which I desire to draw attention, which is this, that common to both the Doctor’s and my own Geissler spectrum I found the before-mentioned rather bright line between γ and α. This line I found no equivalent for in either of the carbon-tubes. For spectra of coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4. [Sidenote: Tube- and flame-spectra of carbon do not correspond.] In comparing the spectra, it should be remembered that the tube- and flame-spectra of carbon do not correspond. Compare, for instance, the spectrum of coal-gas or CO₂ 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 bright yellow 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. 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 shown), see Plate XVI. spectra 6 & 7. [Sidenote: Prof. Piazzi Smyth’s measurements of the components of the citron-band in a coal-gas flame.] _Note._—Prof. Piazzi Smyth has been good enough, at my instance, to measure the components of the citron band of the carbo-hydrogen spectrum (near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged with common air. The spectroscope used had prisms giving 22° of dispersion between A and H, and the observing telescope magnified 10 times. The following is a table of the results communicated to me by the Professor:— Reading of Intensity. Micrometer. Reference line, lithium β 4 16·55 ” sodium, α1 10 18·45 ” ” α2 10 18·51 Citron band. Carbo-hydrogen. Line 1, exquisitely clear 6 21·28 2, ” 5 21·88 3, ” 3 22·44 4, faint but clear 2 22·95 5, faint 1 23·38 6, faint and hazy 1 23·70 7, doubtful ? 23·92 Reference line, thallium α 10 25·08 [Sidenote: Comparison of Dr. Vogel’s O lines and Dr. Watts’s carbon-lines.] From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube as under:—