argued by Leroy that a similar state of involuntary automatic attention, with concomitant diminution or disturbance of voluntary attention, is a necessary condition for the appearance of the visual and auditory hallucinations abnormally experienced in the waking state.[18] There is, then, at the basis of dreaming a seemingly spontaneous procession of dream imagery which is always undergoing transformation into something different, yet not wholly different, from that which went before. It seems a mechanical flow of images, regulated by associations of resemblance, which sleeping consciousness recognises without either controlling or introducing foreign elements. This is probably the most elementary form of dreaming, that which is nearest to waking consciousness, and that in which the peripheral and retinal element of dreaming plays the largest part. The fundamental character of this spontaneous self-evolving procession of imagery is indicated by the significant fact that it tends to take place whenever the more retinal and peripheral part of the visual apparatus is affected by the exhaustion of undue stimulation, or even when the organism generally is disturbed or run down, as in neurasthenic conditions.[19] The most obtrusive and familiar example of visual imagery is furnished by the procession of perpetually shifting and changing after-images which continue to evolve for a considerable time after we have looked at the sun or other brilliant object.[20] Less striking, but more intimately akin to the imagery of dreams, are the hypnagogic visions occurring as we fall asleep, especially after a day during which vision has been unusually stimulated and fatigued, though they do not seem necessarily dependent on such fatigue. Most vivid and instructive of all is the procession of visual imagery evoked by certain drugs. Of these the most remarkable and potent, as well as the best for study, is probably mescal, which happens also to be the only one with which I am myself well acquainted.[21] This substance provokes a constant succession of self-evolving visual imagery which constantly approaches and constantly eludes the semblance of real things; in the earlier stages these images closely resemble those produced by the kaleidoscope, and they change in a somewhat similar manner. Such spontaneous evolution of imagery is evidently a fundamental aptitude of the visual apparatus which many very slightly abnormal conditions may bring into prominence.