Among the problems which the spectroscope has yet failed to solve must be mentioned that of the Aurora Borealis. No doubt something has been learned; but still it must be confessed that the prism has been more successful up to the present in its application to objects which lie like the nebulæ on the very confines of the visible universe, than it has to the aurora, which is, comparatively speaking close at hand. Sir William Huggins gave a summary of our knowledge on this subject. It is certain that the glow of the aurora is in the main due to the effect of electric discharges in the upper regions of the atmosphere. Seeing that we are familiar with the spectra of the atmospheric gases, as produced in our laboratories, it might have been expected that the interpretation of the spectrum of the aurora would be a comparatively easy task. We are still ignorant of the source of the principal line in the green, which, as Huggins remarked, may have an origin independent of that of the other lines.
He also referred to the supposition that the aurora is produced by the dust of meteors; but with reference to this, he noted that experiment has shown that fine metallic dust suspended in gases conveying an electric discharge like that of an aurora will not cause the spectrum to exhibit the characteristic line of the metallic dust in question. There is much to be said for Professor Schuster's suggestion that the principal line in the aurora may be due to some extremely light gas which is present in too small a relative quantity in the lower strata of the atmosphere to permit of its existence being disclosed by spectroscopic or any other form of chemical analysis. In the upper regions where the auroral displays take place,
