Suppose that, when the light has reached the vaporous matter lying at B, an interval of one full minute (much greater than any noticed in Tyndall’s experiments), occurs before the actinic cloud comes into view, a similar interval after the light has passed C before the cloud is seen there, and so on, up to the time of the arrival of the light at Z. Professor Tyndall’s reasoning implied that all the time intervals thus occurring at B, C, D, E, etc., up to Z, had to be added together, to give the total time of the formation of the tail from A to Z, and hence naturally a long time might elapse, and the head having at the end of this time reached a different position from that which it had occupied at the beginning, the divergence of the tail from the direction exactly opposite to the sun, and the curvature of the tail, would be alike readily accounted for. But what are the actual facts of the case. The part of the tail formed latest by the supposed solar actinic action, namely, the part at Z, would be formed just nine minutes after the light had left A, and ten minutes after the part nearest to A had been formed (by the same light waves), for, nine minutes after leaving A, the light would be at Z, and a minute after each epoch (according to our supposition) the actinic cloud would be formed respectively at A and at Z. We get just the same interval—nine minutes—whether the actinic cloud appears immediately after light has traversed the vapour which is to form the cloud, or a minute after, or an hour after. In every case the tail would be formed outwards from A, at the rate at which light travels. This does not accord with the phenomena—in fact, the supposition that a tail could be formed at the rate at which light travels, will be found, on examination, to lead to many most manifest absurdities, which Professor Tyndall doubtless recognized when he sought escape from the supposition of such rapid tail formation, through the effects he attributed to the delayed appearance of the actinic cloud. Another theory in explanation of the rapid formation of such a tail as that of Newton’s comet is worthy of far less notice. Professor Tyndall’s theory was based on an interesting physical fact, which he had himself discovered, and which was also manifestly akin in character to the formation of a comet’s tail. The one to be now noticed was suggested to a mathematician by a rather familiar phenomenon, the effects of which on his imagination he seems to have been never able to entirely overcome—at any rate no amount of evidence against the theory seems to counterbalance in his mind the notion once conceived that the theory might be true. (It is a way some theorists have.)