These flitting lights can be easily imitated. A prudent experimenter ought never to lose sight of the fact, that it is possible to employ diverse substances in order to produce phosphorescent effects. The use of phosphorescent oil, for example, will give fictitious luminous phenomena. I remember a seance at which the medical student, of whom I have already spoken, was present. I noticed that one of his finger-tips shone for a moment. I afterwards learnt, that this young man had a phial of phosphorescent oil in one of his pockets. On another occasion, long narrow lights were, from time to time, seen on his body. I think these were produced by matches or straws dipped in the luminous liquid. Phosphorescent preparations, as a rule, have the advantage of only becoming very luminous, when they are shaken about in the air; for the lights, which are given forth by the phosphorus they contain, are only produced when there are phenomena of oxydation. Objects coated over with sulphide of calcium, strontium, or baryum, become luminous in obscurity, when they have been previously exposed to light. This is the principle of luminous dials, match-boxes and candle-sticks. There are also other substances which permit of simulating luminous phenomena. I was once present at some seances, which were very curious from the point of view of the luminous phenomena which I observed. These seances were of the series of which I have already spoken. The two young tricksters, some of whose misdeeds I have related, were present, and as one of them is an excellent chemist, it is possible that the superb phenomena I observed were not altogether authentic. I confess, I do not see how fraud was committed; but, given the conditions under which I experimented, I think I ought to abstain from expressing a favourable opinion upon the reality of the facts observed. I will describe them succinctly, indicating the phenomena which could have been simulated, and those which did not appear to be so.