Wave-lengths, in hundred-thousandths of a millimetre.
It is unfortunate that in this illustration and in Professor Herschel’s paper the wave-lengths of the Aurora-lines are not given in figures, but must be roughly calculated from the scale. Professor Herschel speaks of Ångström’s drawing as representing a normal spectrum, and as derived from authentic sources, such as Vogel, Barker, and others; but beyond this we are not certain as to its origin.
In illustration of the difficulty of constructing any thing like a general typical Aurora-spectrum I append a Table of eight Auroral spectra taken at hazard:—
Table of compared Aurora.
Auroral lines and bands.
| Observers. | Red. | Yellow. | Green. | Blue. | Indigo. | Violet. | |||
|---|---|---|---|---|---|---|---|---|---|
| Vogel, April 9, 1871 | 6297 | 5569 | 5390 | 5233 | 5189 | 5004 | — | 4694 to 4629 | — |
| Barker, Nov. 9, 1871 | 6230 | 5620 | — | — | 5170 | 5020 | 4820 | — | — |
| Barker, Oct. 14, 1873 | 6300 | 5550 | — | 5330 to 5200 | — | 5050 to 4990 | 4930 to 4850 | 4740 to 4670 | 4310 |
| A. Clarke, junr., Oct. 24, 1870 | 5690 | — | 5320 | — | — | 4850 | — | 4350 | |
| Backhouse, 1873 | 6060 | 5660 | — | — | 5165 | 5015 | — | 4625 | 4305 |
| Backhouse, Feb. 4, 1874 | * | 5570 | — | — | 5180 | 4980 | 4830 | 4640 | 4320 |
| H. R. Procter, 1870 | * | * | — | * | — | — | — | * | * |
| Lord Lindsay, 1870 | — | * | * | * | * | — | — | — | * |
* Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths.
Ångström’s drawing discussed.
On examining Ångström’s diagram it certainly seems to me that, upon the showing of the drawing itself, the coincidences are not very exact. All three of the violet-pole bands appear to be less refrangible than the Aurora-lines with which they are compared-the middle one (at 47) considerably so, the one near E (at about 52·30) appreciably so, and the third (at 43) slightly so.