THE PEECEPTION OP SPACE. (IV.) 529

The mystery is solved when we note the class to which all these experiences belong. They are ' apperceptions ' of definite * things,' definitely situated in tridimensional space. The mind uniformly uses its sensations to identify things ly. The sensation is invariably apperceived by the idea, name or ' normal ' aspect (p. 349) of the thing. The peculiarity of the optical signs of things is their extraordinary mutability. A ' thing ' which we follow with the eye, never doubting of its physical identity, will change its retinal image incessantly. A cross, a ring, waved about in the air, will pass through every conceivable angular and elliptical form. All the while, however, as we look at them, we hold fast to the perception of their ' real ' shape, by mentally combining the pictures momentarily received with the notion of peculiar positions in space. It is not the cross and ring pure and simple which we perceive, but the cross so held, the ring so held. From the day of our birth we have sought every hour of our lives to correct the apparent form of things, and translate it into the real form by keeping note of the way they are placed or held. In no other class of sensa- tions does this incessant correction occur. What wonder, then, that the notion ' so placed ' should invincibly exert its habitual corrective effect, even when the object with which it combines is only an after-image, and make us perceive the latter under a changed but more ' real ' form ? The ' real ' form is also a sensation conjured up by memory ; but it is one so probable, so habitually conjured up when we have just this combination of optical experiences, that it partakes of the invincible freshness of reality, and seems to break through that law which elsewhere condemns repro- ductive processes to being so much fainter than sensations. Once more, these cases form an extreme. Somewhere, in the list of our imaginations of absent feelings, there must be found the vividest of all. These optical reproductions of real form are the vividest of all. It is foolish to reason from cases lower in the scale, to prove that the scale can contain no such extreme cases as these; and particularly foolish since we can definitely see why these imaginations ought to be more vivid than any others, whenever they recall the forms of habitual and probable things. These latter, by incessantly repeated presence and reproduction, will plough deep grooves in the nervous system. There will be de- veloped, to correspond to them, paths of least resistance, of unstable equilibrium, liable to become active in their totality when any point is touched off. Even when the objective stimulus is imperfect, we shall still sec the full convexity of