Mercury-spectrum. How obtained.
This spectrum is given as useful for comparison with the bright and principal Aurora-line. It is easy to obtain with a small coil, the metal being used as one electrode. The yellow lines are distinct and steady; but the green, which is very bright, is apt to flicker as the spark moves on the surface of the metal (see Plate XVI. spectrum 5).
The following Table was compiled for the purpose of comparing the foregoing results with the Aurora-spectrum.
Table of coincidences.
Table showing comparative position of Aurora-lines with the principal lines in the examined spectra. C. means coincident within the limits of my instrument and scale, N. near, and VN. very near.
| Aurora-lines | 6297 β. | 5569 α. | 5390 ζ. | 5233 δ. | 5180 δ. | 5004 γ. | 4694 to 4629 ε. | 4350? ε. |
|---|---|---|---|---|---|---|---|---|
| Hydrogen-tube | No results in the examined spectra; but see Plate XIII. fig. 2. | N. | N. | C., same W.L. | Band includes 2 lines. | Too uncertain in position for comparison (see Plate XIII. fig. 1). | ||
| Coal-gas tube | N. | VN. | Band includes 1 line. | |||||
| Oxygen-tube | VN. | |||||||
| Air, capillary | Band includes | Band includes | N. | Band includes 2 lines. | ||||
| Air, violet-pole | Band includes | C. | Band includes 1 line. | |||||
| Air, red-pole | See Air, capillary. | |||||||
| Aurora-tube and phosphorescent tube | See Air, capillary; and note bright line. | |||||||
| Air, spark | N. | N. | C. | Band includes 2 lines. | ||||
| Air, spark over water | Continuous spectrum and faint air-lines. | Band includes 2 lines. | ||||||
| Phosphoretted hydrogen | N. | Faint band. | Band includes | |||||
| Iron | VN. | N. | VN. | VN. | ||||
Tested by coincidence, or close proximity of lines to those of the Aurora, we arrange the spectra in the following order:—(1) iron, (2) air-spark, (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) carbon and oxygen.
The air-tube spectrum might perhaps stand higher in the scale but for its broad bands, which make comparison doubtful. Lines of oxygen possibly escape detection in the Aurora from the faint character of its spectrum.
The phosphorus and iron spectra are especially interesting in connexion with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546).