Diagram of Vogel’s Aurora and violet-pole spectrum.
As it seemed desirable to adopt a specific Aurora-spectrum for comparison, and to show such comparison on a somewhat larger scale than Ångström’s drawing, I prepared the diagram shown on Plate XI. fig. 1. The upper spectrum is Vogel’s, already described and figured on Plate XIII. The lower spectrum is that of “Air, violet pole,” Plate XV. spectrum 2, graphically shown. I can only find one absolute coincidence in the two compared spectra in this diagram.
It should, too, I think, be borne in mind that there is a great difference in the character of the compared spectra, whether as shown in Ångström’s drawing or mine—the bands of the violet-pole spectrum mostly degrading towards the violet, while the lines or bands of the Aurora in no way possess that character[14].
Dr. Vogel’s violet-pole and Aurora-lines.
To assist in the foregoing violet-pole comparison I add the following Table derived from Dr. Vogel’s memoir:—
| Violet-pole lines. | Aurora-lines. | ||
|---|---|---|---|
| W.L. | W.L. | ||
| 6100, | broad, moderately bright stripe | 6297, | very bright stripe. |
| 5945, | |||
| 5459, | broad, moderately bright stripe | 5569, | brightest line of spectrum. |
| 5289, | 5390, | extremely faint line. | |
| 5224, | very bright line | 5233, | moderately bright. |
| 5147, | faint line | 5189, | moderately bright. |
| 5004, | bright line | 5004, | very bright line. |
| 4912, | fainter than last. | ||
| 4808, | very faint line. | 6694, | band less brilliant in the middle. |
| 4704, | very intense line. | 4663, | |
| 4646, | very faint line. | 4629, | |
| 4569, | moderately bright. | ||
| 4486, | moderately bright. | ||
| 4417, | quite faint line. | ||
| 4346, | moderately bright line. | ||
| 4275, | very bright line. | ||
On examination of these figures it will be seen that 5224 and 5233 are fairly close, and that 5004 is coincident. Beyond these there is little to identify the spectra.
Conclusions arrived at adverse to the violet-pole theory.
As the general result of my observations and a comparison of the foregoing spectra and tables, I see no reason for giving to the violet-pole glow any special or distinguished place in a comparison with the Aurora, and certainly not for assigning to it the nearly absolute monopoly of the spectrum. It is true that the line γ in the violet-pole glow (Plate XV. spectrum 2), which, by the way, degrades towards the red, is in close coincidence with one of the Aurora-lines; but how are the brighter bands α and β accounted for? These, as I have before pointed out, alone survive when the tube is placed at a distance from the slit. It is true they are thus reduced to shaded-off lines in lieu of bands; but the difficulty still remains, that they are conspicuous for their absence in the Aurora-spectrum. On the whole, I cannot but conclude that Professor Ångström’s theory fails. At all events, if the violet-pole glow-spectrum is to represent the Aurora-spectrum, it must be under conditions different from those by which it obtains in dry-air vacuum-tubes or flasks at ordinary temperature.