But now, accepting telepathy as an established fact, the problem remains: How are we to explain it? What is the mechanism by which one person is able to transmit messages directly and instantaneously to another person although they may be half the world apart? To this question, it must frankly be admitted, no positive answer can as yet be returned. But some extremely plausible hypotheses have been advanced, not by mere theorists but by eminent men of science, who, themselves affirming the actuality of telepathy, have given much thought to the problem of its mode of operation. Sir William Crookes, for example, calling attention to the marvelous but undisputed facts of ethereal vibration as evidenced by the phenomena of wireless telegraphy and the Röntgen rays, urges that here we have quite possibly an adequate explanation of the mystery of telepathy on a wholly naturalistic basis--that is to say, a basis which enables us to accept telepathy without dislocating our entire conception of the physical universe. "It seems to me," he suggests, "that in these rays [Röntgen rays] we may have a possible way of transmitting intelligence which, with a few reasonable postulates, may supply the key to much that is obscure in psychical research. Let it be assumed that these rays, or rays of even higher frequency, can pass into the brain and act on some nervous center there. Let it be conceived that the brain contains a center which uses these rays as the vocal chords use sound vibrations (both being under the command of intelligence), and sends them out, with the velocity of light, to impinge on the receiving ganglion of another brain. In this same way the phenomena of telepathy, and the transmission of intelligence from one sensitive to another through long distances, seem to come into the domain of law and can be grasped."[15] [15] Presidential Address to the Society for Psychical Research, January 29, 1897. This undoubtedly is the explanation that most strongly commends itself to those scientists who courageously acknowledge their belief in telepathy. Nor do they see any objection to it in the fact that people apparently are affected by the telepathic impulse only at certain times. For the brain of both sender and receiver may conceivably, on the analogy of wireless telegraphy, be set to transmit and receive telepathic communications only when attuned to vibrations of a certain amplitude. There is, however, as Sir William Crookes himself has recognized, another and really formidable objection to this vibratory hypothesis.