It has been observed that certain people do not obtain phenomena when they operate alone, but obtain them, on the contrary, when with another person. I myself have not had occasion to remark this fact, but I have often noticed that the presence of certain people favoured the attainment of results, while the presence of others troubled or stopped it. I have no explanation to offer for this fact. Certainly credulity or incredulity has no influence whatever on the results of an experiment. I have seen people who were very little inclined to allow themselves to be convinced make excellent auxiliaries. At the same time, I have seen convinced spiritists make detestable co-operators. It seems as if the faculty of giving forth this unknown force were unequally distributed, that it constitutes a physical property of the organism; that, in relation to it, some persons will be positive and others negative, some will emit and others absorb it. Hence the importance of the choice of co-operators—_of the composition of the circle_. The number of experimenters is comparatively unimportant; in principle, the more numerous the circle the greater the force thrown out. But the presence of a large number of sitters is a bad condition for observation; it also enhances the difficulty of the realisation of, what spiritists call, the harmony of the circle. But I ought to say that the finest luminous phenomena, which I have seen, have been obtained when there were from fifteen to twenty people present. On the other hand, I have had the opportunity of experimenting several times alone with a non-professional medium, when I succeeded in seeing faces which I recognised. Unfortunately, this medium—the only one with whom I have obtained this phenomenon—wishes to retain his incognito. I think the most favourable number is from four to eight. I would urge those who wish to try to experiment to compose their circle, as far as possible, of an equal number of each sex; it is preferable to alternate the masculine and feminine elements. These considerations lead us to the examination of methods of operation, properly speaking. III. METHODS OF OPERATION Before discussing in detail those methods which appear to me to be the surest, I think it well to make a few general recommendations. The first relates to the state of mind in which it is necessary to experiment. If interesting results are desired it is not advisable to laugh, joke, or mock at those practices—however ridiculous they may seem—with which I advise compliance. Act seriously, do not make light of experiments, the exact import of which we are so ignorant of. I think we should also avoid the other extreme, which we find in most spiritistic groups, and which impart to these seances all the solemnity of a religious service.