It has been objected to the view just expressed that the whole sky should glow in a nebulous light, and that even the outer atmosphere of the earth should display such a glow. But hydrogen and helium occur only very sparely in the terrestrial atmosphere. We find, however, another light, the so-called auroral line, which may possibly be due to krypton in our atmosphere. Whichever way we turn the spectroscope on a very clear night, especially in the tropics, we observe this peculiar green line. It was formerly considered to be characteristic of the Zodiacal Light, but on a closer examination it has been traced all over the sky, even where the Zodiacal Light could not be observed. One of the objections to our view is therefore unjustified. As regards the other objection, we have to remark that any light emission must exceed a certain minimum intensity to become visible. There may be nebulæ, and they probably constitute the majority, which we cannot observe because the number of electrically charged particles rushing into them is far too insignificant. A confirmation of this view was furnished by the flashing-up of the new star in Perseus on February 21 and 22, 1901. This star ejected two different kinds of particles, of which the one kind travelled with nearly double the velocity of the other. The accumulations of dust formed two spherical shells around the new star, corresponding in every respect to the two kinds of comets’ tails of Bredichin’s first and second classes, which we have sometimes observed together in the same comet (Fig. 35). When these dust particles, on their road, hit against nebular masses, the latter became luminescent, and we thereby obtained knowledge of the presence of large stellar nebulæ of whose existence we previously had not the faintest suspicion. Conditions, no doubt, are similar in other parts of the heavens where “we have not so far discovered any nebulæ—we believe, because of the small number of these charged particles straying about in those parts. On the same grounds we may explain the variability of certain nebulæ which formerly appeared quite enigmatical.” V THE SOLAR DUST IN THE ATMOSPHERE—POLAR LIGHTS AND THE VARIATIONS OF TERRESTRIAL MAGNETISM