When the fullest allowance has been made for all possible explanations we find a considerable number of cases remaining of which no other account can be given than that they are apparitions, due to no ascertained cause, which are perceived by two or more persons simultaneously. That the collective perception proves the objective, or--to use a less ambiguous word--the _material_ existence of the thing perceived, is probably held now by few persons outside the ranks of professed mystics. Apart from the theoretical difficulties of such a hypothesis--difficulties which have by no means been surmounted by the invocation of fixed ether, intercalary vortex rings, space of four dimensions, and other subtler forms of the theory evolved in recent times,--it is to be noted that no facts of any significance have been adduced to support it. There is at present no trustworthy evidence that an apparition has ever been weighed or photographed,[122] or submitted to spectroscopic or chemical analysis. But, indeed, the theory betrays its own origin in a prescientific age; and without formal destruction by argument it has shared in the euthanasia which has overtaken many other pious opinions found inadequate to the facts. The phenomena which it professes to explain are paralleled in all their essential features by other phenomena, for which even its supporters would hardly be rash enough to claim substantial reality; and as the phantasms now to be discussed bear in all points a close resemblance to those already described as occurring to solitary percipients, probably no one who accepts the one class of appearances as hallucinatory will hesitate to accept the other.