Among the most ingenious of these is (or rather was, for I think it is no longer maintained even by its eminent author), Prof. Tyndall’s theory of a comet’s tail as an actinic cloud, generated by the passage of the solar rays through exceedingly tenuous matter after those rays had been in part deprived of their heating power, during their passage through the comet’s head. According to this theory the actinic cloud can not be formed under the heating rays, but so soon as the actinic rays fall on the tenuous matter alone, the cloud is formed,—so that all round the region in which would be the comet’s shadow, there is no luminous cloud, while along that region the cloud exists. The rapidity with which light travels would of course make this explanation absolutely perfect in explaining cometic tails lying always exactly in a straight line directed from the sun, or with their axis so situated. But unfortunately this exceedingly rapid formation of the tail (a tail of ninety million miles in length would be formed in about eight minutes) is more than observation requires or can explain. Prof. Tyndall made a slight oversight in dealing with this part of his theory. Noticing that the actinic cloud, as he called it, is not formed instantly, but after a delay of a few seconds, in his experiments, he reasoned as though it would follow from this that the formation of the actinic cloud behind a comet’s head in space might be a process extending its action in distance from the head at a rate considerably less than that at which light travels, yet still fast enough to account for the exceedingly rapid formation of the tail of Newton’s comet, and of other similar tails. But a little consideration will show that the few seconds following the fall of light on the vapors dealt with by Tyndall, before the luminous cloud appeared, would produce no such effect as he imagined. The rate of formation of the tail would still be that at which light travels. Imagine the head at A, for the sake of argument, and the sun’s light after reaching A, passing on to B, C, D, E, etc., to Z, a distance say of one hundred million miles, in nine minutes: A ... B ... C ... D ... E ... ... Z.