Mr. Backhouse’s graphical Spectra of four Auroræ.
Mr. Backhouse’s graphical spectra of Auroræ.
Mr. Backhouse has been good enough to supply me with some details of four several Auroræ seen by him at Sunderland, accompanied by drawings, showing in a graphical way the spectrum of each display as seen with a spectroscope with rather a wide slit and as drawn by eye. I have reduced the four drawings to the same scale, and in this way they are extremely interesting for comparison (Plate V. fig. 4). The line on the left in each spectrum is Ångström’s bright Auroral line, and is supposed to be considerably prolonged. The height of the lines denotes intensity.
April 18, 1873.
April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which Mr. Backhouse had not perceived before. No. 5 had not been visible lately, and Mr. Backhouse thought it must belong to Auroræ of a different type from those which had appeared latterly.
Feb. 4, 1874.
February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw seven lines, all that he had ever seen. (The red line, not shown in the diagram, makes the seventh.)
The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr. Backhouse had only once before seen No. 4, and it became quite invisible between 7.45 and 7.55, though the other lines were as bright as before and the red line had appeared.
Oct. 3, 1874.
October 3rd, 1874. This spectrum was examined, and diagram made between 10 and 10.25 P.M. Five lines only are indicated.