ADDITION  TO  THE   PRKCK1HNU   I'AI'KK.
[For a long time, in fact from before l«S<*7, I have taken a different view of the nature of the phenomenon from that indicated in Arts. 63 and a few following. I was led to it by a phenomenon I casually noticed in making observations on tin* fluorescence of various glasses. The sun's light was reflected horizontally into a darkened room, passed through a convex lens, and with or without previous filtration by passing through a blue glass phuvd at or near the focus, was admitted into tin* glass to bo examined. In general the fluorescence when examined in this way showed no appreciable duration, but in one ela^s of glasses when tin* glass was merely moved sideways by hand somewhat rapidly across thr incident pencil, a luminous trail wan seen extending liom the focus at the moment into the part of I lie glass on which the focus had previously fallen. The colour of the fluorexvnf enne as a l|'                           whole when the glass was at rest, and that-, of the rout of the trail
(or part nearest to the focus) when the glass was in motion, varied with the composition of the glass, but the distribution of the, fluorescence in the spectrum when the glass was examined in an intense and fairly pure speetrum was the same or v««ry nearly so, and was that characteristic distribution alluded to in § 7.S. 1 may mention that I have circumstantial evidence that thr Mihstanee in the glass to which this effect was due was mangani^-r in a lower state of oxidation than that, which gives the purple e.ilour. No\\ the colour of the trail changed from its root to its end, becoming reddish at last, in such a manner as to indicate a decreasing nn-an refrangibility, except in one case, that, of a phovphatie -la-prepared by the Rev. W. Vernon-Harcourt, where the colour was red to start with, so that there was no opportunity to change.
The change of colour with the lapse of time since excitement of the glass led me to regard the alteration in the molecular disturbance as brought about in the following manner. The