But apart from the consideration of theories such as those, either demonstrably untenable, though ingenious, like Professor Tyndall’s, or altogether and obviously untenable like Professor Tait’s, there are certain phenomena of comets’ tails which force upon us the belief that they are phenomena of repulsion, though the repulsive action is of a kind not yet known to physicists. 1. The curvature of all the cometic tails when not seen from a point in or near the place of their motion. 2. The existence of more tails than one to the same comet, the different tails being differently curved. 3. The phenomena of striations athwart the tail. It is evident that all these phenomena are such as we might fairly expect if a comet’s tail is caused by the sun’s repulsive action on molecules, raised by his heating action on the head. The matter thus swept away would resemble smoke, driven upwards from the funnel of a moving steamer, and then swept in any given direction by a steady wind; we should see a curved train of such matter just as we see a curved streak of smoke. If the matter raised from the head is not all of one kind (and it is antecedently unlikely that it should be), there would be more than one trail of matter, if the sun’s repulsive action were different on these different kinds of matter. Lastly, the striations seen athwart the tail, as in the well known case of Donati’s great comet, would be explained, either as due to the observed pulsational manner in which the envelopes are raised (if matter were raised uniformly from the head there could be no formation of successive envelopes), or else as due to the carrying off into the main tail, where alone such striations are seen, of matter which, had it freed itself at the beginning, would have been swept off into the smaller tails, but being as it were entangled in the great outflow of matter forming the large tail, escapes later, and when it does, gets swept off at its own more rapid rate, and there forms a streak lying at an angle with the direction of the principal tail. Bredichin has shown that where there are three tails to a comet, their forms correspond with the theory that the envelopes raised from the head are principally formed of hydrogen, carbon and iron, but this, which, if established, would be the most important physical discovery yet made respecting comets, seems open at present to considerable doubt, though confirmations seem to be given to it, in some respects, by the results of spectroscopic analysis. To spectroscopic analysis we must in all probability look for such information respecting comets, as may hereafter enable us to understand their nature. On this point let us consider what is said by one who, if not the greatest living astronomical spectroscopist, is _facile princeps_ in this country—Dr. W. Huggins. First, however, we must consider the past of this method of research as applied to comets.