sort re-presentations of the original impression, but a sequent presentation of diametrically opposite quality; while positive after-sensations are, psychologically regarded, nothing but the original sensations in a state of evanescence. It is this continuance and gradual waning after the physical stimulus has completely ceased that give after-sensations their chief title to a place in the transition from impression to image. There is, however, another point: after-sensations are less affected by movement than impressions are. If we turn away our eyes we cease to see the flame at which we have been looking, but the after-image remains still projected before us and continues localized in the dark field of sight, even if we close our eyes altogether. This fact that movements do not suppress them, and the fact that yet we are distinctly aware of our sense-organs being concerned in their presentation, serve to mark off after-sensations as intermediate between primary and secondary presentations. The after-sensation is in reality more elementary than either the preceding percept or its image. In both these, in the case of sight, objects appear in space of three dimensions, 'i.e. with all the marks of solidity and perspective# but the so-called after-image lacks all these.
Still further removed from normal sensations (i.e. sensations determined by the stimuli appropriate to the sense-organ) are the “recurrent sensations” often unnoticed but probably experienced more or less frequently by everybody-cases, that is, in which sights or sounds, usually such as at the time were engrossing and impressive, suddenly reappear several hours or even days after the physical stimuli, as well as their effects on the terminal sense-organ, seem entirely to have ceased. Thus workers with the microscope often see objects which they have examined during the day stand out.clearly before them in the dark; it was indeed precisely such an experience that led the anatomist Henle first to call attention to these facts. But he and others have wrongly referred them to what he called a “ sense-memory ”; all that we know is against the supposition that the eye or the ear has any power to retain and reproduce percepts. “ Recurrent sensations ” have all the marks of percepts which after-sensations lack; they only differ from what are more strictly called “ hallucinations ” in being independent of all subjective suggestion determined by emotion or mental derangement.
In what Fechner has called the “ memory after-image ” or the primary memory-image, as it is better termed, we have the image proper in its earliest form. As an instance of what is meant may be cited the familiar experience that a knock at the door, the hour struck on the clock, the face of a friend whom we have passed unnoticed, may sometimes be recognized a few minutes later by means of the persisting image, although-apparently the actual impression was entirely disregarded. But in vision the primary memory-image can always be obtained, and is obtained to most advantage, by looking intently at some object for an instant and then closing the eyes or turning them away. The image of the object will appear for a moment very vividly and distinctly, and can be so recovered several times in succession by an effort of attention. Such reinstatement is materially helped by rapidly opening and closing the eyes, or by suddenly moving them in any way. 'In this respect a primary memory image resembles an after-sensation, which can be repeatedly revived in this manner when it would otherwise have disappeared. This seems to show that the primary memory-image in such cases The following scant quotation from Fechner, one of the best observers in this department, must suffice in illustration. “ Lying awake in the early morning after daybreak, with my eyes motionless though open, there usually appears, when I chance to close them for a moment, the black after-image of the white bed immediately before me and the white after-image of the black stove-pipe some distance away against the opposite wall .... Both [after images] appear as if they were in juxtaposition in the same plane; and, though-when my eyes are open-I seem to see the white bed in its entire length, the after-image-when my eyes are shut-presents instead only a narrow black stripe owing to the fact that the bed is seen considerably foreshortened. But the memory-image on the other hand completely reproduces the pictorial illusion as it appears when the eyes are open" (Elemanu der Psvchobhysik, ii. 473). owes its vivacity in part to a positive after-sensation; at any rate it proves that it is in some way still sense-sustained. But in other respects the two are very different: the after-sensation is necessarily presented if the intensity of the original excitation suffices for its production, and cannot be presented otherwise, however much we attend. Moreover, the after-sensation is only for a moment positive, and then passes into the negative or complementary phase, when, so far from even contributing towards the continuance of the original percept, it directly hinders it. Primary memory-images on the other hand, and indeed all images, depend mainly upon the attention given to the impression; provided that was sufficient, the faintest impression may be long retained, and without it very intense ones will soon leave no trace. The primary memory-image retains so much of its original definiteness and intensity as to make it possible with great accuracy to compare two physical phenomena, one of which is in this way “ remembered ” while the other is really present. For the most part this is indeed a more accurate procedure than that of dealing with both together, but it is only possible for a very short time. From Weber's experiments with weights and lines” it would appear that even after ro seconds a considerable waning has taken place, and after 100 seconds all that is distinctive of the primary image has probably ceased.
On the whole, then, it appears that the ordinary memory image is a joint effect; it is not the mere residuum of changes in the presentation-continuum, but an effect of these only when there has been some concentration of attention upon them. It has the form of a percept, but is not constituted of revived impressions, for the essential marks of impressions are absent; there is no localization in, or projection into, external space, neither is there the motor adaptation, nor the tone of feeling, incident to the reception of impressions. Ideas do not reproduce the intensity of these original constituents, but only their quality and complication. What we call the vividness of an idea is of the nature of intensity, but it is an intensity very partially and indirectly determined by that of the original impression; it depends much more upon the state of what we shall call the memory-continuum and the attention the idea receives. The range of vividness in ideas is probably comparatively small; what are called variations in vividness are often really variations in distinctness and completeness? Where we have great intensity, as in hallucinations, primary presentations may be reasonably supposed to enter into the complex. It is manifest that the memory-continuum has been in someway formed out of or differentiated from the presentation continuum by the movements of attention, but the precise connexion of the two continua is still very difficult to determine. We see perhaps the first distinct step of this evolution in the primary memory-image: here there has been no cessation in presentation, and yet the characteristic marks of the impression are gone, so much so, indeed, that superposition without “fusion ” with an exactly similar impression is possible. We have now to inquire into the genesis and development of ideation. V Genesis and Developmenl of Ideation.
2 3. We find ourselves sometimes engrossed in present perceptions, as when tracing, for example, the meanderings of an ant; at other times we may be equally absorbed in reminiscences; or, again, in pure reverie and “ castle-building.” Here are three well-marked forms of conscious life: the first being concerned with what is, the second with what has been, and the third with the merely possible. Again, the first involves definite spatial and temporal order, though the temporal order, as just said, is in the main restricted to the “ sensible present ”; the second involves only definite time-order; and the last neither in a definite way. Thus, analytically regarded, perception, memory, imagination, show a steady advance. In infancy the first 2 Die Lehre vom Tastsinne, &c., pp. 86 seq.
3 As we have seen that there is a steady transition from percept to image, so, if space allowed, the study of hallucinations might make clear an opposite and abnormal process-the passage, that is to say, of 1ITlQ.g€S ll'lt0 percepts, for such, to all intents and purposes, are hallucinations of perception, psychologically regarded.