The head of the micrometer-screw is divided into 100 parts, and each part, in the neighbourhood of the Fraunhofer line F, answers to about ·00016 wave-length. The probable error of position on one of the well-marked lines in the sun’s spectrum amounts to about 0·008 of a turn of the screw with the lowest eyepiece of the telescope. I have subjected the screw itself to a thorough examination with reference to such range, as well as to periodical inequalities in the single worms of the screw, but could discover no error exceeding 0·01 of a turn of the screw. I have to mention, further, that after each observation in the position in which the instrument was used, readings followed on the sodium-lines, or on some of the hydrogen-lines, in order to eliminate errors which might arise in the unavoidable disturbance of any particular part of the spectral apparatus.
1. Observations of the Aurora.
1870, Oct. 25th.—A very bright Aurora. In the brightest parts, besides a very bright line between D and E, several other fainter lines were to be discerned, situated further towards the blue end of the spectrum. They appeared on a dimly-lighted ground, and stretched out over the Fraunhofer lines E and b to about midway between b and F. Towards the red end the spectrum was terminated by the bright line first mentioned. No measurements could be taken, as the apparatus had not yet undergone the above-mentioned alterations, and even the brightest line of the spectrum did not diffuse sufficient light to be able to perceive the fine cross wires. The red rays of the Aurora were not examined.
1871, Feb. 11th.—Towards ten o’clock appeared in the north-west a very bright light-bow of greenish colour as the edge of a dark segment. Even with a very narrow slit, the line between D and E could be well recognized and measured. The average of six readings gave 7·11 turns, equal to 5572 wave-length. In a small spectroscope of low dispersion which is arranged on Browning’s plan, a few more lines placed further towards the blue could be recognized (as in October). Towards the red end of the spectrum no lines were observable. The greatest development of the Aurora was about midnight. Magnificent rays rose to about 60° elevation; they had the same greenish colouring as the bow of light, and the appearance of the spectrum also was exactly the same. I again obtained two sets of measurements: the average of six readings in the first set gave 7·10 turns, 5572 wave-length; in another part of the heavens at the same time 7·10 was the result of four readings.
On Feb. 12, towards eight o’clock, the intensity of the Aurora was already great enough to allow measurements of the brightest line. The average of six readings gave 7·09 turns, or 5576 wave-length. Dr. Lohse took observations later, with the same apparatus, and found from six readings 7·12 turns, or 5569 wave-length.
Yet the appearance of the spectrum in the spectroscope of low dispersion was essentially distinct from that of February 11th. The green continuous spectrum was present; it extended from the bright Aurora-line to the lines b of the solar spectrum, and was traversed by some bright lines. Between band b and F, was another line standing alone, out beyond F, in the blue part of the spectrum, a clear bright stripe; and just before G a very faint broad band of light was perceived.
Amongst the rays which, later on, shot upwards, and were coloured red at their ends, another very intense red line appeared in the spectrum between C and D, yet placed nearer to C[20].
April 9th.—An exceedingly brilliant Aurora, of which the greater development took place in the early morning hours. Magnificent red sheaths rose up to the zenith. The spectrum was like that observed on February 12th, only much more intense, so that the lines could be seen and measured with the larger spectral apparatus. In the brightest part of the Aurora was the dark segment; the spectrum consisted of five lines in the green, and a somewhat indistinct broad line or band in the blue.
The red rays, on the other hand, allowed us to recognize seven lines, whilst the bright line again appeared in the red part of the spectrum. I could not again perceive the faint stripe observed on February 12th, in the vicinity of line G. The mean measurements of four readings on an average, for each line, gave:—
| Turns of screw. | Probable errors. | Wave-length. | Probable errors. | Remarks. | |
|---|---|---|---|---|---|
| 4·62 | ·0037 | 6297 | ·00014 | Very bright stripe. | On a faintly lighted ground. |
| 7·12 | 9 | 5569 | 2 | Brightest line of the spectrum; becomes noticeably fainter at appearance of the red line. | |
| 7·92 | — | 5390 | — | Extremely faint line; unreliable observation. | |
| 8·71 | 21 | 5233 | 4 | Moderately bright. | |
| 8·95 | 49 | 5189 | 9 | This line is very bright when the red line appears at the same time, otherwise equal in brilliancy with the preceding one. | |
| 10·06 | 20 | 5004 | 3 | Very bright line. | |
| 12·33 | — | 4694 | — | Broad band of light, somewhat less brilliant in the middle. | |
| 12·59 | 22 | 4663 | — | ||
| 12·88 | — | 4629 | 3 | Very faint in those parts of the Aurora in which the red line appears. | |