Pulsations.—Do pulsations travel in any invariable direction? What time do they take to get from one part of the heavens to another? Are they running sheets of continuous light, or fixed patches which become luminous, or more luminous, in rapid succession? and if patches, do these appear to be foreshortened streamers? Are the same patches luminous in successive pulsations?
Sounds (?).—As some have suspected the Aurora to be accompanied by sound, the observer’s attention should be directed to this question when an Aurora is seen during a calm. If sound be suspected, the observer should endeavour, by changing his position, brushing off spicules of ice from the neighbourhood of the ears, his whiskers, &c., to ascertain whether it can be referred to the action of such wind as there is on some part of his dress or person. If it should clearly appear that it is not referable to the wind, then the circumstance of its occurrence, its character, its relation (if any) to bursts of light, should be most carefully noted.
These questions are prepared merely to lead the observer to direct his attention to various features of the phenomenon. Answers are not demanded, except in such cases as definite answers can be given; and the observer should keep his attention alive to observe and regard any other features which may appear to be of interest. It is desirable that drawings should be made of remarkable displays.
Observations with Sir William Thomson’s electrometer would be very interesting in connexion with the Aurora, especially a comparison of the readings before, during, and after a passage of the Aurora across the zenith.
Spectroscopic Observations. By Prof. G. G. Stokes, Sec. R.S.
Spectrum of the Aurora.
The spectrum of the Aurora contains a well-known conspicuous bright line in the yellowish green, which has been accurately observed. There are also other bright lines of greater refrangibility, the determination of the positions of which is more difficult on account of their faintness, and there are also one or more lines in the red, in red auroras.
Advantage should be taken of an unusually bright display to determine the positions of the fainter lines. That of the brightest lines, though well known, should be measured at the same time to control the observations. The character of the lines (i. e. whether they are strictly lines, showing images of the apparent breadth of the slit, or narrow bands, sharply defined or shaded-off) should also be stated.
Sometimes a faint gleam of light is seen at night in the sky, the origin of which (supposed from the presence of clouds) is doubtful. A spectroscope of the roughest description may in such cases be usefully employed to determine whether the light is auroral or not, as in the former case the auroral origin is detected by the chief bright line. The observer may thus be led to be on the look-out for a display which otherwise might have been missed.