I found this plan most effective for the rapid and correct recording of a faint and evanescent spectrum, and it gave close results when compared with traversing-micrometer measured spectra. The records, too, admitted of subsequent examination at leisure. [Sidenote: Double-slit plate arrangement.] Mr. Browning subsequently constructed for me a double-slit plate (lately in the Scientific Loan Collection at South Kensington) for the same instrument (Plate X. fig. 5). The lower half of the plate is fixed. The upper half traverses the lower by the aid of a micrometer-screw. The slit is widened or closed at pleasure by loosening the small screws by which the jaw-plates are attached. A scale is engraved on the fixed lower half of the plate for an approximate measurement, while the division of the micrometer-screw-head completes it. In use, one half of the spectrum slides along the other, and a bright line in the upper spectrum is selected as an index. The distances between the lines of the lower half of the spectrum are read off by means of the bright line above. This form of micrometer was suggested by Mr. Procter (in ‘Nature’) many years ago as a substitute for a more complicated apparatus by Zöllner. Other instruments on a similar principle have been lately introduced, but for Aurora purposes I prefer a fixed scale. [Sidenote: Photographed spectrum suggested.] 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. [Sidenote: 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._ [Sidenote: 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.