These visions may be _hypnopompic_ as well as _hypnagogic_;--may appear, that is to say, at the moment when slumber is departing as well as at the moment when it is coming on;--and in either case they are closely related to dreams; the "hypnagogic illusions" or pictures being sometimes repeated in dream (as with Maury), and the hypnopompic pictures consisting generally in the persistence of some dream-image into the first moments of waking. In either case they testify to an intensified power of inward visualisation at a very significant moment;--a moment which is actually or virtually one of sleep, but which yet admits of definite comparison with adjacent moments of waking. We may call the condition one of cerebral or "mind's eye" hyperæsthesia,--an exalted sensibility of special brain-centres in response to those unknown internal stimuli which are always giving rise to similar but fainter inward visions even in broadly waking hours. For those who are already good visualisers such phenomena as these, though striking enough, present no quite unique experience. For bad visualisers, on the other hand, the vividness of these hypnagogic pictures may be absolutely a revelation. The degree of acuteness, not of the visualising faculty alone, but of all the senses in dream, is a subject for direct observation, and even--for persons who can at all control their dreams--for direct experiment. Some correspondents report a considerable apparent accession of sensory power in dream. Others again speak of the increased vividness of dramatic conception, or of what has been called in a hypnotic subject "objectivation of types." "In each of these dreams," writes one lady, "I was a man;--in one of them a low brute, in the other a dipsomaniac. I never had the slightest conception of how such persons felt or thought until these experiences." Another correspondent speaks of dreaming two disconnected dreams,--one emotional and one geometrical,--simultaneously, and of consequent sense of confusion and fatigue. The "Chapter on Dreams," in R. L. Stevenson's volume, _Across the Plains_ (already referred to in the last chapter), contains a description of the most successful dream-experiments thus far recorded. By self-suggestion before sleep Stevenson could secure a visual and dramatic intensity of dream-representation which furnished him with the motives for some of his most striking romances. His account, written with admirable psychological insight, is indispensable to students of this subject. I am mentioning these well-known phenomena, as the reader will understand, with a somewhat novel purpose--to show, namely, that the internal sensory perceptions or imaginative faculty of sleep may exceed that of vigilance in something the same way as the recuperative agency of sleep surpasses the _vis medicatrix_ of waking hours.