The frigate, the Belle Poule, was cruising in the open sea for the purpose of finding the cruiser Le Berceau, from which she had been separated by a violent storm. It was broad daylight and in full sunshine. Suddenly the watch signalled a disabled vessel; the crew looked in the direction signalled, and every one, officers and sailors, clearly perceived a raft covered with men towed by boats which were displaying signals of distress. Yet this was nothing more than a collective hallucination. Admiral Desfosses lowered a boat to go to the rescue of the wrecked sailors. On nearing the object sighted, the sailors and officers on board the boat saw "masses of men in motion, stretching out their hands, and heard the dull and confused noise of a great number of voices." When the object was reached those in the boat found themselves simply and solely in the presence of a few branches of trees covered with leaves that had been swept out from the neighbouring coast. Before evidence so palpable the hallucination vanished.
The mechanism of a collective hallucination of the kind we have explained is clearly seen at work in this example. On the one hand we have a crowd in a state of expectant attention, on the other a suggestion made by the watch signalling a disabled vessel at sea, a suggestion which, by a process of contagion, was accepted by all those present, both officers and sailors.
It is not necessary that a crowd should be numerous for the faculty of seeing what is taking place before its eyes to be destroyed and for the real facts to be replaced by hallucinations unrelated to them. As soon as a few individuals are gathered together they constitute a crowd, and, though they should be distinguished men of learning, they assume all the characteristics of crowds with regard to matters outside their speciality.