And now, standing once more upon our watershed of entencephalic vision, let us trace the advancing capacities of internal vision. The forms of vision now to be considered are virtually independent of the eye; they can persist, that is to say, after the destruction of the eye, if only the eye has worked for a few years, so as to give visual education to the brain. We do not, in fact, fully know the limits of this independence, which can only be learnt by a fuller examination of intelligent blind persons than has yet been made. Nor can we say with certainty how far in a seeing person the eye is in its turn influenced by the brain. I shall avoid postulating any "retropulsive current" from brain to retina, just as I have avoided any expression more specific than "the brain" to indicate the primary seat of sight. The arrangement here presented, as already explained, is a psychological one, and can be set forth without trespassing on controverted physiological ground. We may take _memory-images_ as the simplest type of internal vision. These images, as commonly understood, introduce us to no fresh knowledge; they preserve the knowledge gained by conscious gaze upon the outer world. In their simplest spontaneous form they are the _cerebral_ sequelæ of external vision, just as after-images are its _entoptic_ sequelæ. These two classes of vision have been sometimes confounded, although the distinction is a marked one. Into the cerebral storage of impressions one element habitually enters which is totally absent from the mere retinal storage, namely, a psychical element--a rearrangement or generalisation of the impressions retinally received. Next we come to a common class of memory-images, in which the subliminal rearrangement is particularly marked. I speak of _dreams_--which lead us on in two directions from memory-images; in the direction of _imagination-images_, and in the direction of _hallucinations_. Certain individual dreams, indeed, of rare types point also in other directions which later on we shall have to follow. But dreams as a class consist of confused memory-images, reaching a kind of low hallucinatory intensity, a glow, so to say, sufficient to be perceptible in darkness. I will give the name of _imagination-images_ to those conscious recombinations of our store of visual imagery which we compose either for our mere enjoyment, as "waking dreams," or as artifices to help us to the better understanding of facts of nature confusedly discerned. Such, for instance, are imagined geometrical diagrams; and Watt, lying in bed in a dark room and conceiving the steam-engine, illustrates the utmost limit to which voluntary internal visualisation can go.