Such indications have not been wanting. In the chapter on Genius, and in the chapter on Sleep, we have traced the existence of many classes of these pictures; all of them ready, as it would seem, to manifest themselves on slight inducement. _Dream-figures_ will rise in any momentary blur of consciousness; _inspirations_ will respond to the concentrated desire or the mere passing emotion of the man of genius; _after-images_ will recur, under unknown conditions, long after the original stimulus has been withdrawn; _memory-images_ will surge up into our minds with even unwished-for vividness; the brilliant exactness of _illusions hypnagogiques_ will astonish us in the revealing transition from waking to sleep. All is prepared, so to say, for some empirical short-cut to a fuller control of these subjacent pictures; just as before Mesmer and Puységur all was prepared for an empirical short-cut to trance, somnambulism, suggestibility. All that we want is to hit on some simple empirical way of bringing out the correlation between all these types of subjacent vision, just as mesmerism was a simple empirical way of bringing out the correlation between various trances and sleep-waking states. _Crystal-vision_, then, like hypnotic trance, might have been gradually evolved by a series of reasoned experiments, along an unexceptionable scientific road. In reality, of course, this prehistoric practice must have been reached in some quite different way. It does not fall within the scope of this book to trace the various streams of divination which converge into Dr. Dee's magic, and "the attracting of spirits into the ball." But it is really to the Elizabethan Dr. Dee--one of the leading _savants_ of his time--that the credit must be given of the first systematic attempt to describe, analyse, and utilise these externalised pictures.[104] I will describe briefly the general type of the experiment, and we shall see how near we can get to a psychological explanation.