Plate XI.
Spectrum of the Aurora Australis.
Captain Maclear’s spectra of Aurora Australis.
Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874 (antè, p. 27), with the spectroscope, found three prominent lines in the yellow-green, green, and blue or purple, but not the red line. In the Aurora of March 3rd, 1874 (p. 27), he could trace four lines, three bright and one rather faint. They must have been exceedingly bright to show so plainly in full-moon light.
Instrument used, and mode of registering lines.
The spectroscope used was a Grubb single-prism with long collimator. A needle-point in the eyepiece marked the position of the lines; and a corresponding needle-point, carried on a frame by a screw movement in concord with the point of the eyepiece, scratched the lines on a plate of blackened glass. Two plates were taken. On the first were scratched the auroral lines and the solar lines as seen in the moonlight; on the second plate were scratched the auroral lines, the Solar lines from the moon, and the carbon lines in a spirit-lamp.
Copies of the two spectra obtained. Discrepancy in the spectra. Remarks on the spectra.
The next morning the solar lines were verified in sunlight. I subjoin (Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the auroral lines being marked A, the solar lines by the usual designating letters, and the carbon by Car. To these spectra I have added for comparison Dr. Vogel’s spectrum of the Aurora Borealis. Captain Maclear could not account for the different positions of the auroral lines in the two plates; for the prism, as far as he was aware, was not moved during the observations. As the solar lines are indicated in the same place in both spectra, the case would seem one of actual change of position of the auroral lines during observation. A comparison of the two spectra gives the impression that the lower one is the same as the upper, except that the dispersion is greater, the lines remaining relatively in position. One does not, however, see how the dispersion could have so varied in a single-prism instrument, and the position of the solar lines is adverse to such an explanation.
There is a suspicion that Auroræ are not always identical in position of some of the lines; but the line in the green (considerably out of place in the lower Australis spectrum) has always, within small limits, the same position. It will be noticed how much further the Australis spectrum runs into the violet than Vogel’s Borealis, the latter having no lines much beyond F.
The faint line (No. 2) mentioned by Captain Maclear possibly corresponds with Dr. Vogel’s band. The absence of the four lines of the Aurora Borealis in the green part of the spectrum of the Australis is peculiar; and in this respect, too, the two Australis spectra agree.