In this instrument the reference-prism is brought into action by turning the slipping piece to which is fixed the two terminals. Care should be taken that the prism itself is adjusted before commencing observations, as it may be shaken out of position on the voyage. The tubes provided for the reference-spectra may be either fastened to the terminals or arranged in some other manner. The air-spectrum may also be used as a reference-spectrum. To get this, two wires should be screwed into the insulators, their ends being at such a distance apart and in such a position that the spectrum is well seen. _General Observations regarding the Spectrum of the Aurora[17]._ _C._ Note appearance, colour, &c. of _arc_, _streamers_, _corona_, and _patches_ of light. Get compass positions of principal features, and _note any change of magnetic intensity_. If corona forms, take its position and apparent height. Look out for _phosphorescence_ of Aurora and adjacent clouds. Listen for reported sounds. Note any peculiarity of cloud scenery, prior to or pending the Aurora. Sketch principal features of the display, and indicate on this sketch the parts spectroscopically examined. Examine line in _red_ specially in reference to its assumed connexion with _telluric_ lines (little _a_ group), and note _as to its brightening in sympathy with any of the other lines_. Examine line in yellow-green (Ångström’s) as to _brightness_, _width_, and _sharpness_ (_or nebulosity_) at the edges. Notice as to a peculiar _flickering_ in this line sometimes seen; note also whether this line is _brighter_ (or the reverse) _with a fall of temperature_. Note _ozone_ papers at the time of Aurora. Note whether the Auroræ can by their spectra be classed into distinct types or forms, and examine for _different spectra_ as under:— α. The auroral _glow_, pure and simple. β. The white arc. γ. The streamers and corona. δ. Any phosphorescent or other patches of light, or light cloud in or near the Auroræ. _D._ The information collected together in the ‘Manual’ should be carefully consulted, and the line of observations suggested by Ångström’s later work followed out. To do this, not only record the positions of any features you may observe in the spectrum, but endeavour to determine, if any, and if so which, of the features vary together. Compare, for instance, the two spectra of nitrogen in the Geissler tube supplied, by observing first the narrow and then the wider parts of the tube. It will be seen that the difference in colour and spectrum results simply from an addition to the spectrum in the shape of a series of channelled spaces in the more refrangible end in the case of the spectrum of the narrow portion. Try to determine whether the difference between red and green Auroras may arise from such a cause as this, and which class has the simpler spectrum.