The sun, then, acts on these bodies when they approach him, produces in them important physical and chemical transformations, and exercises on their developed atmosphere a repulsive force, the nature of which is still unknown to us, but the effects of which coincide with the formation and development of the tails. The tails are thus, in the extension of the cometary atmosphere, driven back, either by the solar heat, by the light, by electricity, or by other forces; and this extension is rather a motion in the ether than a real transport of matter, at least in the great comets which approach very near the sun, and in their immense luminous appendages. The effects produced and observed are not the same in all comets, which proves that they differ from each other in several respects. The tails have sometimes been seen to diminish before the perihelion passage, as in 1835. Luminous envelopes have also been seen succeeding each other round the head, concentrating themselves on the side opposite to the sun, and leaving the central line of the tail darker than the two sides. This is what happened in the Donati comet and in that of 1861. Sometimes a secondary tail has been seen projected towards the sun, as in 1824, 1850, 1851, and 1880. Comets have been seen with the head enveloped in phosphorescence, surrounding them with a sort of luminous atmosphere. Comets have also been seen with three, four, five, and _six tails_, like that of 1744, for example, which appeared like a splendid aurora borealis rising majestically in the sky, until, the celestial fan being raised to its full height, it was perceived that the six jets of light all proceeded from the same point, which was nothing else but the nucleus of a comet. On the other hand, the nuclei themselves show great variations--some appear simply nebulous, and permit the faintest stars to be visible through them; others seem to be formed of one or more solid masses surrounded by an enormous atmosphere; in others, again, a nucleus does not exist, as in the Southern comet of 1887. One of the comets of 1888 showed a triple nucleus and a bristling coma, as may be seen in the cut. We may, then, consider that the wandering bodies collected under the name of comets are of several origins and _several different species_. _Why_ the tails of comets should so persistently avoid the sun it is impossible to say. We do not even know whether the cause lies actually in the sun or in the comet. Astronomers speak of the “repulsive energy” with which the sun “sweeps away” from his neighborhood the light vapory matter of which the tails are made. But the how and the wherefore of this strange seeming repulsion they can not explain. For at the selfsame time the sun appears to be attracting the comet towards himself, and driving the comet’s tail away from himself.