In ‘Photographed Spectra’ I have pointed out that we shall probably obtain no spectrum of the Aurora to be absolutely depended upon for comparison with other spectra until we succeed in a photographed one. From experiments made with a special prism of the Rutherfurd form, constructed for me by Mr. Browning (with which many gas-spectra have been already photographed), I see no reason, should an unusually bright Aurora favour us with a visit, why its spectrum may not be recorded in a permanent form, and with lines sufficiently well marked to be compared with other spectra. Rapid dry plates would be especially useful for such a purpose, and some Auroræ, if wanting in brilliancy, would doubtless compensate by their period of endurance.
Mr. Hilger’s half-prism spectroscope.
Mr. Adam Hilger has also made for me one of his “half-prism” spectroscopes, in which considerable dispersion is obtained with but very little loss of light. This instrument has a simple and rapid micrometer arrangement, with a bright line as an index. I have (for want of Auroræ) had no opportunity of trying it, but I doubt not it is well adapted for such a purpose.
Spectrum of the Aurora described.
Lines or bands and continuous spectrum.
The spectrum of the Aurora consists of a set of lines or bands upon a dark ground at each extremity of the spectrum, but with more or less of faint continuous spectrum towards the centre. The extreme range of the spectrum, as observed up to the present time, is from “a” (between C and D) in the red to “h” (hydrogen) in the violet.
Lines nine in number.
The lines have been classified and arranged by Lemström and others as nine in number, but I believe not more than seven have ever been seen simultaneously.
The author of the article “Aurora Polaris,” in the ‘Encyclopædia Britannica,’ classes the lines as nine, and gives a table with the following results (to these I have added Herr Vogel’s lines, for the purpose of identification and comparison):—