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.
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.
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.
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.
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:—
| Intensity. | Reading of 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 | |