It is mainly distinguished from the two preceding spectra by the brightness of the continuous spectrum on which the lines 2, 3, and 4 lie, and by the weakness of No. 5.
Oct. 4, 1874.
October 4th, 1874. Taken between 11.10 and 11.20 P.M.; distinguished, like the last, by a considerable amount of continuous spectrum and by a faint line (No. 3), not seen in the last spectrum, while No. 3 in the last is missing in this spectrum.
Mr. Backhouse’s remarks as to comparative frequency of some of the Auroral lines.
Mr. Backhouse, as to both these last spectra, remarks that the lines were very variable in intensity, and sometimes some were visible and sometimes others. They varied also in relative brightness in different parts of the sky at the same time. Mr. Backhouse, in a communication to ‘Nature,’ referring to a statement of Mr. Procter’s, that the bands of the Auroral spectrum are seldom visible, except the bright line at 5570, says that he always found two bands, “doubtless Winlock’s 4640 and 4310,” to be invariably visible when the Aurora was bright enough to show them. Of thirty-four Auroræ examined by Mr. Backhouse, fourteen showed the lines 4640 and 4310, and three others at least one of these, while eight showed the red line. (Ångström only once saw this line.) In five Auroræ, all more or less red, he saw a faint band, the wave-length of which he placed at 5000 or 5100. He never saw the line 5320 (also Winlock’s coronal line), unless it were once, probably from want of instrumental power. With regard to these observations, I may say that with a Browning’s miniature spectroscope I saw only two lines (the red and the green) in the grand display of the 24th October, 1870; and with an instrument of larger aperture the green line only on the 4th February, 1872; while I saw the green line and three others towards the violet with the same instrument during the Aurora of 4th February, 1874. (See description of this Aurora, antè p. 21, and drawing of spectrum, Plate VI. fig. 1 a.)
Lord Lindsay’s Aurora-Spectrum, 21st October, 1870.
Lord Lindsay’s Aurora of 21st Oct., 1870.
Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on the night of the 21st October, 1870. It commenced about 9.30, reached its maximum about 11, and faded away suddenly about 11.30 P.M.
Spectrum described.
A spectrum obtained in the north-west gave five bright lines with a Browning’s direct-vision spectroscope—two strong, one medium, two very faint. A tallow candle was used to obtain a comparison spectrum of sodium and carburetted hydrogen.