"A direct collision of two celestial bodies is not regarded by Huggins as an admissible explanation of the Nova; a partial collision has little probability, and the most that can be admitted is perhaps the mutual penetration and admixture of the outer gaseous envelopes of the two bodies at the time of their closest approach. A more probable explanation is given by an hypothesis which we owe to Klinkerfues, and which has more recently been further developed by Wilsing, viz. that by the very close passage of two celestial bodies enormous tidal disturbances are produced and thereby changes in the brightness of the bodies. In the case of the two bodies which form the Nova, it must be assumed that these phenomena are displayed in the highest degree of development, and that changes of pressure have been produced which have caused enormous eruptions from the heated interior of the bodies; the eruptions are perhaps accompanied by electrical actions, and are comparable with the outbursts in our own sun, although they are on a much larger scale."[337] It will be noticed that this hypothesis agrees with the fundamental assumption of the "Planetesimal Hypothesis" advocated by Professors Chamberlin and Moulton (see my _Astronomical Essays_, p. 324). The rush of a comparatively small body through a mass of gaseous matter seems also a very plausible hypothesis. This idea was originally advanced by Prof. Seeliger, and independently by Mr. Monck. With reference to the nebula which was observed round the great new star of 1901--Nova Persei--Prof. Lewis Bell supports the theory of Seeliger, which accounts for the apparent movements of the brightest portions of the nebula by supposing that the various parts of the highly tenuous matter were successively lighted up by the effects of a travelling electro-magnetic wavefront, and he shows that this theory agrees well with the observed phenomenon.[338] The "collision theory" which explained the sudden outburst of light by the meeting of two dark bodies in space, seems to be now abandoned by the best astronomers. The rapid cooling down of the supposed bodies indicated by the rapid decrease of light is quite inconsistent with this hypothesis. The rapid diminution in the light of some of these "new stars" is very remarkable. Thus the new star which suddenly blazed out near the nucleus of the great nebula in Andromeda in August, 1885, faded down in 5 months from "the limit of visibility to the naked eye to that of a 26-inch telescope"! A _large_ body could not cool in this way. Mr. Harold K. Palmer thinks that the "complete and astonishingly rapid changes of spectral type observed in the case of _Nova Cygni_ and _Nova Aurigæ_, and likewise those observed in _Nova Normæ_, _Nova Sagittarii_ and _Nova Persei_, leave little doubt that the masses of these objects are small."[339]