The day Auroræ, which are elsewhere described, and are not very uncommon, could, we may presume, hardly be seen without the presence of some phosphorescent glow. Professor Ångström, in his Aurora Memoir (discussed elsewhere), in discussing the yellow-green line, considers the only probable explanation to be that it owes its origin to fluorescence or phosphorescence. He says that some fluorescence is produced by the ultra-violet rays; and adds, “an electric discharge may easily be imagined, which, though in itself of feeble light, may be rich in ultra-violet light, and therefore in a condition to cause a sufficiently strong fluorescent light.” And he refers to the fact that oxygen and some of its compounds are phosphorescent. [Sidenote: A phosphoretted hydrogen spectrum-band is close to yellow-green auroral line. Phosphorescent or fluorescent after-glow of electric discharge.] In the examination of certain spectra connected with the Aurora, detailed in Part II., I have shown that the bright edge of one of the phosphoretted hydrogen bands is in close proximity to the yellow-green Auroral line. I have also referred to the peculiar brightening by reduction of temperature of one of the bands in the red end of the spectrum of phosphoretted hydrogen, so that from almost invisible it became bright, and to the peculiar brightening of a line in the yellow-green in certain “Aurora” and phosphorescent tubes. It has also been observed that the electric discharge has a phosphorescent or fluorescent after-glow (isolated, I believe, by Faraday). It seems difficult to avoid in some way connecting all these circumstances with the yellow-green line of the Aurora, if not also with the line in the red. [Sidenote: Sorby’s experiments on fluorescence and absorption. Bonelleine, spectrum of. Coloured layer of fungi. Spectrum of _Oscillatoriæ_.] Mr. Sorby, in his experiments on the connexion between fluorescence and absorption (‘Monthly Microscopical Journal’), found in the spectrum of a solution in alcohol of a strongly fluorescent substance called bonelleine (the green colouring-matter found in the _Aurelia Bonellia-viridis_) two bright bands, the one red and the other green, with centres respectively at 6430 and 5880, and their limits towards the blue end at 6320 and 5820. On adding an acid the red band changed its place to 6140. The superficial membranous coloured layer of the fungi _Russula nitida_ and _vesca_ in alcohol gave an absorption band with centre at 5540, while the spectrum of fluorescence extended to 4400. A solution of _Oscillatoriæ_ in water gave a spectrum of absorption with bands at 6200 and 5690; while the spectrum of fluorescence showed two bright bands having their centres at 6470 and 5800, and their limits towards the blue end at 6320 and 5710. [Sidenote: Sea phosphorescence, a continuous spectrum.]