The Principles of Psychology, Vols. 1-2

* I ought to say that I seem always able to see the cross rectangular at will. But this appears to come from an imperfect absorption of the rectangular after-image by the inclined plane at which the eyes look. The cross, with me, is apt to detach itself from this and then look square. I get the illusion better from the circle, whose after-image becomes in various ways elliptical on being projected upon the different surfaces of the room, aad cannot then be easily made to look circular again.

late it into the real form by keeping note of the way they are placed or held. In no other class of sensations 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 reproductive 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 reproduxitions 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 developed, 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 see the full convexity of a human face, the correct inclination of an angle or sweep of a curve, or the distance of two lines. Our mind will be like a polyhedron, whose facets are the attitudes of perception in which it can most easily rest. These are worn upon it by habitual objects, and from one of these it can pass only by tumbling over into another.*

Hering has well accounted for the sensationally vivid character of these habitually reproduced forms. He says, * In Chapter XVIII, p. 74, I gave a reason why imaginations ought not to be as vivid as sensations. It should be borne in mind that that reason does not apply to these complemental imaginings of the real shape of things actually before our eyes. after reminding us that every visual sensation is correlated to a physical process in the nervous apparatus :

** If this psychophysical process is aroused, as usually happens, by light-rays impinging on the retina, its form depends not only on the nature of these rays, but on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds itself. The same stimulus may excite widely different sensations according to this state. " The constitution of the nervous apparatus depends naturally in part upon innate predisposition ; but the ensemble of effects wrought by stimuli upon it in the course of life, whether these come through the eyes or from elsewhere, is a co-factor of its development. To express it otherwise, involuntary and voluntary experience and exercise assist in determining the material structure of the nervous organ of vision, and hence the ways in which it may react on a retinal image as an outward stimulus. That experience and exercise should be possible at all in vision is a consequence of the reproductive power, or memory, of its nerve-substance. Every particular activity of the organ makes it more suited to a repetition of the same ; ever slighter touches are required to make the repetition occur. The organ habituates itself to the repeated activity. . . .

" Suppose now that, in the first experience of a complex sensation produced by a particular retinal image, certain portions were made the special objects of attention. In a repetition of the sensible experience it will happen that notwithstanding the identity of the outward stimulus these portions will be more easily and strongly reproduced ; and when this happens a hundred times the inequality with which the various constituents of the complex sensation appeal to consciousness grows ever greater.

"Now in the present state of our knowledge we cannot assert that in both the first and the last occurrence of the retinal image in question the same pure sensation is provoked, but that the mind interprets it differently the last time in consequence of experience ; for the only giveri things we know are on the one hand the retinal image which is both times the same, and on the other the mental percept which is both times different ; of a third thing, such as a pure sensation, interpolated between image and percept, we know nothing. We ought, therefore, if we wish to avoid hypotheses, simply to say that the nervous apparatus reacts the last time differently from the first, and gives us in consequence a different group of sensations.

' ' But not only by repetition of the same retinal image, but by that of similar ones, will the law obtain. Portions of the image common to the successive experiences will awaken, as it were, a stronger echo in the nervous apparatus than other portions. Hence it results that reproduction is usually elective : the more strongly reverberating parts of the picture yield stronger feelings than the rest. This may result in the latter being quite overlooked and, as it were, eliminated from perception. It may even come to pass that instead of these parts eliminated by election a feeling of entirely different elements comes to consciousnesselements not objectively contained in the stimulus. A group of sensations, namely, for which a strong tendency to reproduction has become, by frequent repetition, ingrained in the nervous system will easily revive as a whole when, not its whole retinal image, but only an essential part thereof, returns. In this case we get some sensations to which no adequate stimulus exists in the retinal image, and which owe their being solely to the reproductive power of the nervous apparatus. This is complementary {ergdnzende) reproduction.

"Thus a few points and disconnected strokes are sufficient to make us see a human face, and without specially directed attention we fail to note that we see much that really is not drawn on the paper. Attention will show that the outlines were deficient in spots where we thought them complete. . . . The portions of the percept supplied by complementary reproduction depend, however, just as much as its other portions, on the reaction of the nervous apparatus upon the retinal image, indirect though this reaction may, in the case of the supplied portions, be. And so long as they are present, we have a perfect right to call them sensations, for they differ in no wise from such sensations as correspond to an actual stimulus in the retina. Often, however, they are not persistent ; many of them may be expelled by more close observation, but this is not proved to be the case with all. . . . In vision with one eye . . . the distribution of parts within the third dimension is essentially the work of this complementary reproduction, i.e. of former experience. . . . When a certain way of localizing a particular group of sensations has become with us a second nature, our better knowledge, our judgment, our logic, are of no avail. . . . Things actually diverse may give similar or almost identical retinal images; e.g., an object extended in three dimensions, and its flat perspective picture. In such cases it often depends on small accidents, and especially on our will, whether the one or the other group of sensations shall be excited. . . . We can see a relief hollow, as a mould, or vice versd; for a relief Illuminated from the left can look just like its mould illuminated from the right.

Reflecting upon this, one may infer from the direction of the shadows that one has a relief before one, and the idea of the relief will guide the nerve-processes into the right path, so that the feeling of the relief is suddenly aroused. . . . Whenever the retinal image is of such a nature that two diverse modes of reaction on the part of the nervous apparatus are, so to speak, equally, or nearly equally, imminent, it must depend on small accidents whether the one or the other reaction is realized. In these cases our previous knowledge often has a decisive effect, and helps the correct perception to victory. The bar© idea of the right object is itself a feeble reproduction which with the help of the proper retinal picture develops into clear and lively sensation. But if there be not already in the nervous apparatus a disposi

tion to the production of that percept which our judgment tells us is right, our knowledge strives in vain to conjure up the feeling of it ; we then know that we see something to which no reality corresponds, but we see it all the same." * Note that no object not probable, no object which we are not incessantly praxitised in reprodticing, can acquire this vividness in imagination. Objective corners are ever changing their angles to the eyes, spaces their apparent size, lines their distance. But by no transmutation of position in space does an objective straight line appear bent, and only in one position out of an infinity does a broken line look straight. Accordingly, it is impossible by projecting the after-image

of a straight line upon two surfaces which make a solid angle with each other to give the line itself a sensible *kink.' Look with it at the corner of your room: the after-image, which may overlap all three surfaces of the corner, still continues straight. Yolkmann constructed a complicated surface of projection like that drawn in Fig. 77, but he found it impossible so to throw a straight afterimage upon it as to alter its visible form. One of the situations in which we oftenest see things is spread out on the ground before us. We are incessantly dirilled in making allowance for this perspective, and reducing things to their real form in spite of optical foreshortening. Hence if the preceding explanations are true, we ought to find this habit inveterate. The lower half of the retina, which habitually sees the farther half of things spread out on the ground, ought to have acquired a habit of enlarging its pictures by imagination, so as to make them more than equal to those which fall on the upper retinal surface ; and this habit ought to be hard to escape from, even when both halves of the object are equidistant from the eye, as in a vertical line on paper. Delboeuf has found, accordingly, that if we try to bisect such a line we place the point of division about -^-^ of its length too high.*

Similarly, a square cross, or a square, drawn on paper, should look higher than it is broad. And that this is actually the case, the reader may verify by a glance at Fig. 78. For analogous reasons the upper and lower halves of the letter S, or of the figure 8, hardly seem to differ. But when turned upside down, as g, g, the upper half looks much the larger, t f Wundt seeks to explain all these illusions by the relatively stronger * feeling of innervation ' needed to move the eyeballs upwards, — a careful study of the muscles concerned is taken to prove this, — and a consequently greater estimate of the oistance traversed. It suflSces to remark, however, with Lipps, that were the innervation all, a column of S's placed on top of each other should look each larger than the one below it, and a weathercock on a steeple gigantic, neither of which is the case. Only the halves of the same object look different in size, because the customary correction

Hering has tried to explain our exaggeration of small angles in the same way. We have more to do with right angles than with any others : right angles, in fact, have an altogether unique sort of interest for the human mind. Nature almost never begets them, but we think space by means of them and put them everywhere. Consequently obtuse and acute ones, liable always to be the images of right ones foreshortened, particularly easily revive right ones in memory. It is hard to look at such figures as a, J, c, in Fig. 79, without seeing them in perspective, as

approximations, at least, to foreshortened rectangular forms. * At the same time the genuine sensational form of the lines before us can, in all the cases of distortion by suggested perspective, be felt correctly by a mind able to abstract from the notion of perspective altogether. Individuals differ in this abstracting power. Artistic training improves it, so that after a little while errors in vertical bisection, in estimating height relatively to breadth, etc., become impossible. In other words, we learn to take the optical sensation before us pure, f

for foreshortening bears only on the relations of the parts of special things spread out before us. Cf. Wundt, Physiol. Psych., 3te Aufl. ii. 96-8; Th. Lipps, Grundtatsachen, etc., p. 535. * Hering would partly solve in this way the mystery of Pigs. 60, 61, and 67. No doubt the explanation partly applies ; but the strange cessation of the illusion when we fix the gaze fails to be accounted for thereby. f Helmholtz has sought (Physiol. Optik, p. 715) to explain the divergence of the apparent vertical meridians of the two retinae, by the manner in which an identical line drawn on the ground before us in the median plane will throw its images on the two eyes respectively. The matter is too technical for description here ; the unlearned reader may be referred for it to J. Le Conte's Sight in the Internat. Scient. Series, p. 198 ff. But, for the benefit of those to whom verbum sat, I cannot help saying that it seems to me that the escactness of the relation of the two meridians — whether dlTer-

We may then sum up our study of illusions by saying that they in no wise undermine our view that every spatial determination of things is originally given in the shape of a sensation of the eyes. Thej only show how very potent certain imagined sensations of the eyes may become. These sensations, so far as they bring definite forms to the mind, appear to be retinal exclusively. The movements of the eyeballs play a great part in educating our perception, it is true ; but they have nothing to do with constituting any one feeling of form. Their function is limited to exciting the various feelings of form, by tracing retinal streaks ; and to comparing them, and measuring them off against each other, by applying different parts of the retinal surface to the same objective thing. Helmholtz's analysis of the facts of our * measurement of the field of view ' is, bating a lapse or two, masterly, and seems to prove that the movements of the eye have had some part in bringing our sense of retinal equivalencies about — equivalencies ^ mind, of different retinal forms and sizes, not forms and sizes themselves. Superposition is the way in which the eyemovements accomplish this result. An object traces the line AB on a peripheral tract of the retina. Quickly we move the eye so that the same object traces the line ah on a central tract. Forthwith, to our mind, AB and ah are judged equivalent. But, as Helmholtz admits, the equivalence-judgment is independent of the way in which we may feel the form and length of the several retinal pictures themselves :

"The retina is like a pair of compasses, whose points we apply in succession to the ends of several lines to see whether they agree or not in length. All we need know meanwhile about the compasses is that the distance of their points remains unchanged. What that distance is, and what is the shape of the compasses, is a matter of no account."* gent or not, for their divergence differs in individuals and often in one individual at diverse times— precludes its being due to the mere habitual falling-off of the image of one objective line on both. Le Conte, e.g., measures their position down to a sixth of a degree, others to tenths. This indicates an organic identity in the sensations of the two retinas, which the experience of median perspective horizontals may roughly have agreed with, but hardly can have engendered. Wundt explains the divergence eui usual, by the Innervatiomgefuhl {op. cit. ii. 99 ff.). * Physiol. Optik, p. 547.

Mecisurement implies a stuff to measure. Retinal sensations give tJi£, stuff; objective things form the yard-stick ; motion does the measuring operation; which can, of course, be well performed only where it is possible to make the same object fall on many retinal tracts. This is practically impossible where the tracts make a wide angle with each other. But there are certain directions in the field of view, certain retinal lines, along which it is particularly easy to make the image of an object slide. The object then becomes a * ruler' for these lines, as Helmholtz puts it,* making them seem straight throughout if the object looked straight to us in that part of them at which it was most distinctly seen.

But all this need of superposition shows how devoid of exact space-import the feelings of movement are per se. As we compare the space-value of two retinal tracts by superposing them successively upon the same objective line, so we also have to compare the space-value of objective angles and lines by superposing them on the same retinal tract. Neither procedure would be required if our eye-movements were apprehended immediately, by pure muscular feeling or innervation, for example, as distinct lengths and directions in space. To compare retinal tracts, it would then suffice simply to notice how it feels to move any image over them. And two objective lines could be compared as well by moving different retinal tracts along them as by laying them along the same. It would be as easy to com-

* " We can with a short ruler draw a line as long as we please on a plane surface by first drawing one as long as the ruler permits, and then sliding the ruler somewhat along the drawn line and drawing again, etc. If the ruler is exactly straight, we get in this way a straight line. If it is somewhat curved we get a circle. Now, instead of the sliding ruler we use in the field of sight the central spot of distinctest vision impressed with a Knear sensation of sight, which at times may be intensified till it becomes an after-image. We follow, in looking, the direction of this line, and iu 80 doing we slide the line along itself and get a prolongation of its length. On a plane surface we can carry on this procedure on any sort of a straight or curved ruler, but in the field of vision there is for each direction and movement of the eye only one sort of line which it is possible for us to slide along in its own direction continually. " These are what Helmholti calls the ' circles of direction ' of the visual field — lines which he has studied with his usual care. Cf. Physiol. Optik, p. 548 ff.

pare non-parallel figures as it now is to judge of those which are parallel.* Those which it took the same amount of movement to traverse would be equal, in whatever direction the movement occurred. With this we may end our long and, I fear to many readers, tediously minute survey. The facts of vision form a jungle of intricacy ; and those who penetrate deeply into physiological optics will be more struck by our omissions than by our abundance of detail. But for students who may have lost sight of the forest for the trees, I will recapitulate briefly the points of our whole argument from the beginning, and then proceed to a short historical survey, which will set them in relief.

All our sensations are positively and inexplicably extensive wholes. The sensations contributiug to sipace-perception seem exclusively to be the surface of skin, retina, and joints. * Muscular ' feelings play no appreciable part in the generation of our feelings of form, direction, etc. The total bigness of a cutaneous or retinal feeling soon becomes subdivided by discriminative attention. Movements assist this discrimination by reason of the peculiarly exciting quality of the sensations which stimuli moving over surfaces arouse.

Subdivisions, once discriminated, acquire definite relations of position towards each other within the total space. These * relations' are themselves feelings of the subdivisions that intervene. When these subdivisions are not the seat of stimuli, the relations are only reproduced in imaginary form. The various sense-spaces are, in the first instance, incoherent with each other ; and primitively both they and their subdivisions are but vaguely comparable in point of bulk and form.

The education of our space-perception consists largely of two processes — reducing the various sense-feelings to a common measure, and adding tkem together into the single all-including space of the real world. Both the measuring and the adding are performed by the aid of things. The imagined aggregate of positions occupied by all the actual or possible, moving or stationary, things which we know, is our notion of ^real' space — a very incomplete and vague conception in all minds.

The measuring of our space-feelings against each other mainly comes about through the successive arousal of different ones by the same thing, by our selection of certain ones as feelings of its real size and shape, and by the degradation of others to the status of being merely signs of these. For the successive application of the same thing to different space-giving surfaces motion is indispensable, and hence plays a great part in our space-education, especially in that of the eye. Abstractly considered, the motion of the object over the sensitive surface would educate us quite as well as that of the surface over the object. But the selfmobility of the organ carrying the surface accelerates immensely the result.

In completely educated space-perception, the present sensation is usually just what Helmholtz (Physiol. Optik, p. 797) calls it, * a sign, the interpretation of whose meaning is left to the understanding.' But the understanding is exclusively reproductive and never productive in the process ; and its function is limited to the recall of previous space-sensations with which the present one has been associated and which may be judged more real than it.

Finally, this reproduction may in the case of certain visual forms be as vivid, or almost so, as actual sensation is. The third dimension forms an original element of all our space-sensations. In the eye it is subdivided by various discriminations. The more distant subdivisions are often shut out altogether, and, in being suppressed, have the effect of diminishing the absolute space-value of the total field of view.* * This shrinkage and expansion of the absolute spacevalue of the total optical sensation remains to my mind the most obscure part of the whole

Let US now close with a brief historical survey. The first achievement of note in the study of space-perception was Berkeley's theory of vision. This undertook to establish two points, first that distance was not a visual but a tactile form of consciousness, suggested by visual signs ; secondly, that there is no one quality or ' idea ' common to the sensations of touch and sight, such that prior to experience one might possibly anticipate from the look of an object anything about its felt size, shape, or position, or from the touch of it anything about its look.

In other words, that primitively chaotic or semi-chaotic condition of our various sense-spaces which we have demonstrated, was established for good by Berkeley ; and he bequeathed to psychology the problem of describing the manner in which the deliverances are harmonized so as all to refer to one and the same extended world. His disciples in Great Britain have solved this problem after Berkeley's own fashion, and to a great extent as we have done ourselves, by the ideas of the various senses suggesting each other in consequence of Association. But, either because they were intoxicated with the principle of association, or because in the number of details they lost their general bearings, they have forgotten, as a rule, to state under what sensible form the primitive spatial experiences are found which later became associated with so many other sensible signs. Heedless of their master Locke's precept, that the mind can frame unto itself no one new simple idea, they seem for the most part to be trying to explain the extensive quality itself, account for it, and evolve it, by the mere association together of feelings which originally possessed it not. They first evaporate the nature of extension by making it tantamount to mere * coexistence,' and then they explain coexistence as being the same thing as succession, provided it