The Principles of Psychology, Vols. 1-2

retinal impression. Prepare a sheet with the figures shown in Fig. 57 strongly marked upon it, and get by direct fixation a distinct after-image of each. Project the after-image of the cross upon the upper lefthand part of the wall, it will appear as in Fig. 58 ; on the upper right-hand it will appear as in Fig. 59. The circle similarly projected will be distorted into two different ellipses. If the two parallel lines be projected upon the ceiling or floor far in front, the farther ends will diverge ; and if the three parallel lines be thrown on the same surfaces, the upper pair will seem farther apart than the lower. Adding certain lines to others has the same distorting effect. In what is known as Zollner's pattern (Fig. 60), the long parallels tip towards each other the moment we draw the short slanting lines over them yet their retinal images

are the same they always were. A similar distortion of parallels appears in Fig 61. Drawing a square inside the circle (Fig. 52) gives to the outline of the latter an indented appearance where the square's corners touch it. Drawing the radii inside of one of the right angles in the same figure makes it seem larger than the other. In Fig. 63, the retinal image of the space between the extreme dots is in all three lines the same, yet it seems much larger the moment it is filled up with other dots.

In the stereoscope certain pairs of lines which look single under ordinary circumstances immediately seem double when we add certain other lines to them.* These facts show the indeterminateness of the spaceimport of various retinal impressions. Take now the eye's movements, and we find a similar vacillation. When we follow a moving object with our gaze, the motion is * voluntary ' ; when our eyes oscillate to and fro after we have made ourselves dizzy by spinning around, it is * reflex ' ; and when the eyeball is pushed with the finger, it is * passive.' Now, in all three of these cases we get a feeling from the movement as it effects itself. But the objective perceptions towtich the feeling assists us are by no means the same. In the first case we may see a stationary field of view with one moving object in it ; in the second, the total field swimming more or less steadily in one direction ; in the third, a sudden jump or twist of the same total field.

The feelings of convergence of the eyeballs permit of the same ambiguous interpretation. When objects are near we converge strongly upon them in order to see them ; when far, we set our optic axes parallel. But the exact degree of convergence fails to be felt ; or rather, being felt, fails to tell us the absolute distance of the object we are regarding. Wheatstone arranged his stereoscope in such a way that the size of the retinal images might change without the convergence altering ; or conversely, the convergence might change without the retinal image altering. Under these circumstances, he says,t the object seemed to approach or recede in the first case, without altering its size , in the second, to change its size without altering its distance — just

the reverse of what might have been expected. Wheatetone adds, however, that * fixing the attention ' converted each of these perceptions into its opposite. The same perplexity occurs in looking through prismatic glasses, which alter the eyes' convergence. We cannot decide whether the object has come nearer, or grown larger, or both, or neither ; and our judgment vacillates in the most surprising way. We may even make our eyes diverge, and the object will none the less appear at a finite distance. When we look through the stereoscope, the picture seems at no determinate distance. These and other facts have led Helmholtz to deny that the feeling of convergence has any very exact value as a distance-measurer. ■'^

With the feelings of accommodation it is very much the same. Bonders has shown f that the apparent magnifying power of spectacles of moderate convexity hardly depends at all upon their enlargement of the retinal image, but rather on the relaxation they permit of the muscle of accommodation. This suggests an object farther off, and consequently a much larger one, since its retinal size rather increases than diminishes. But in this case the same vacillation of judgment as in the previously mentioned case of convergence takes place. The recession made the object seem larger, but the apparent growth in size of the object now makes it look as if it came nearer instead of receding. The effect thus contradicts its own cause. Everyone is conscious, on first putting on a pair of spectacles, of a doubt whether the field of view draws near or retreats. :f

There is still another deception, occurring in persons who have had one eye-muscle suddenly paralyzed. This deception f Anomalies of Accommodation and Refraction (New Sydenham Soc. TransL, London, 1864), p. 155. X These strange contradictions have been called by Aubert ' secondary ' deceptions of judgment. See Grundzuge d. Physiologischen Optik (Leipzig, 1876), pp. 601, 615, 627. One of the best examples of them is the small size of the moon as first seen through a telescope. It is larger and brighter, so we see its details more distinctly and judge it nearer. But because we judge it so much nearer we think it must have grown smaller. Cf . Charpentier in Jahresbericht, x. 430.

has led Wundt to affirm that the eyeball-feeling proper, the incoming sensation of effected rotation, tells us only of the direction of our eye-movements, but not of their whole extent."'^ For this reason, and because not only Wundt, but many other authors, think the phenomena in these partial paralyses demonstrate the existence of a feeling of innervation, a feeling of the outgoing nervous current, opposed to every afferent sensation whatever, it seems proper to note the facts with a certain degree of detail.

Suppose a man wakes up some morning with the external rectus muscle of his right eye half paralyzed, what will be the result ? He will be enabled only with great effort to rotate the eye so as to look at objects lying far off to the right. Something in the effort he makes will make him feel as if the object lay much farther to the right than it really is. If the left and sound eye be closed, and he be asked to touch rapidly with his finger an object situated towards his right, he will point the finger to the right of it. The current explanation of the * something ' in the effort which causes this deception is that it is the sensation of the outgoing discharge from the nervous centres, the * feeling of innervation,' to use Wundt 's expression, requisite for bringing the open eye with its weakened muscle to bear upon the object to be touched. If that object be situated 20 degrees to the right, the patient has now to innervate as powerfully to turn the eye those 20 degrees as formerly he did to turn the eye 30 degrees. He consequently believes as before that he has turned it 30 degrees ; until, by a newly-acquired custom, he learns the altered spatial import of all the discharges his brain makes into his right abducens nerve. The * feeling of innervation,' maintained to exist by this and other observations, plays an immense part in the space-theories of certain philosophers, especially Wundt. I shall elsewhere try to show that the observations by no means warrant the conclusions drawn from them, and that the feeling in question is probably a wholly fictitious entity, t Meanwhile it suffices to point out that even those who set most store by it are compelled, by the

readiness with which the translocation of the field of view becomes corrected and further errors avoided, to admit that the precise space-import of the supposed sensation of outgoing energy is as ambiguous and indeterminate as that of any other of the eye-feelings we have considered hitherto. I have now given what no one will call an understatement of the facts and arguments by which it is sought to banish the credit of directly revealing space from each and every kind of eye-sensation taken by itself. The reader will confess that they make a very plausible show, and most likely wonder whether my own theory of the matter can rally from their damaging evidence. But the case is far from being hopeless ; and the introduction of a discrimination hitherto unmade will, if I mistake not, easily vindicate the view adopted in these pages, whilst at the same time it makes ungrudging allowance for all the ambiguity and illusion on which so much stress is laid by the advocates of the intellectualist-theory.

We have native and fixed optical space-sensations ; hut experience leads us to select certain ones from among them to be the exclusive hearers of reality : the rest hecome mere signs and suggesters of these. The factor of selection, on which we have already laid so much stress, here as elsewhere is the solving word of the enigma. If Helmholtz, Wundt, and the rest, with an ambiguous retinal sensation before them, meaning now one size and distance, and now another, had not contented themselves with merely saying : — The size and distance are not this sensation, they are something beyond it which it merely calls up, and whose own birthplace is afar —in * synthesis ' (Wundt) or in * experience ' (Helmholtz) as the case may be ; if they had gone on definitely to ask and definitely to answer the question. What are the size and distance in their proper selves ? they would not only have escaped the present deplorable vagueness of their spacetheories, but they would have seen that the objective spatial attributes * signified ' are simply and solely certain

other optical sensations noio absent, but which the present sensations suggest. What, for example, is the slant-legged cross which we think we see on the wall when we project the rectangular after-image high up towards our right or left (Figs. 58 and 59) ? Is it not in very sooth a retinal sensation itself ? An imagined sensation, not a felt one, it is true, but none the less essentially and originally sensational or retinal for that, — the sensation, namely, which we should receive if a ' real ' slant-legged cross stood on the wall in front of us and threw its image on our eye. That image is not the one our retina now holds. Our retina now holds the image which a cross of square shape throws when in front, but which a cross of the slant-legged pattern ivould throw, provided it were actually on the wall in the distant place at which we look. Call this actual retinal image the ' square ' image. The square image is then one of the innumerable images the slant-legged cross can throw. Why should another one, and that an absent one, of those innumerable images be picked out to represent exclusively the slant-legged cross's * true ' shape ? Why should that absent and imagined slant-legged image displace the present and felt square image from our mind? Why, when the objective cross gives us so many shapes, as it varies its position, should we think we feel the true shape only when the cross is directly in front? And when that question is answered, how can the absent and represented feeling of a slant-legged figure so successfully intrude itself into the place of a presented square one ?

Before answering either question, let us be doubly sure about our facts, and see how true it is that in our dealings tvith objects tve always do pick out on^ of the visual images they yield, to constitute the real form or size. The matter of size has been already touched upon, so that no more need be said of it here. As regards shape, almost all the retinal shapes that objects throw are perspective * distortions.' Square table-tops constantly present two acute and two obtuse angles ; circles drawn on our wallpapers, our carpets, or on sheets of paper, usually show like ellipses ; parallels approach as they recede ; human bodies

are foreshortened ; and the transitions from one to another of these altering forms are infinite and continual. Out of the flux, however, one phase always stands prominent. It is the form the object has when we see it easiest and best : and that is when our eyes and the object both are in what may be called the normal position. In this position our head is upright and our optic axes either parallel or symmetrically convergent ; the plane of the object is perpendicular to the visual plane ; and if the object is one containing many lines it is turned so as to make them, as far as possible, either parallel or perpendicular to the visual plane. In this situation it is that we compare all shapes with each other ; here every exact measurement and decision is made.*

It is very easy to see why the normal situation shovJd have this extraordinary pre-eminenxie. First, it is the position in which we easiest hold anything we are examining in our hands ; second, it is a turning-point between all right- and all left-hand perspective views of a given object ; third, it is the only position in which symmetrical figures seem symmetrical and equal angles seem equal ; fourth, it is often that starting-point of movements from which the eye is least troubled by axial rotations, by which superposition t of the retinal images of different lines and different parts of the same line is easiest produced, and consequently by which the eye can make the best comparative measurements in its sweeps. All these merits single the normal position out to be chosen. No other point of view offers so many aesthetic and practical advantages. Here we believe we see the object as it is ; elsewhere, only as it seems. Experience and custom soon teach us, however, that the seeming appearance passes into the real one by continuous gradations. They teach us, moreover, that seeming and being may be strangely interchanged. Now a real circle may slide into a seeming ellipse ; now an ellipse may, by sliding in the same direction, become a seeming circle ; now

* The only exception seems to be when we expressly wish to abstract from particulars, and to judge of the general 'effect.' Witness ladies trying on new dresses with their heads inclined and their eyes askance ; or painters in the same attitude judging of the * values ' in their pictures. a rectangular cross grows slant-legged ; now a slant-legged one grows rectangular. Almost any form in oblique vision may be thus a derivative of almost any other in * primary ' vision ; and we must learn, when we get one of the former appearances, to translate it into the appropriate one of the latter class ; we must learn of what optical * reality ' it is one of the optical signs. Having learned this, we do but obey that law of economy or simplification which dominates our whole psychic life, when we attend exclusively to the * reality ' and ignore as much as our consciousness will let us the ' sign ' by which we came to apprehend it. The signs of each probable real thing being multiple and the thing itself one and fixed, we gain the same mental relief by abandoning the former for the latter that we do when we abandon mental images, with all their fluctuating characters, for the definite and unchangeable Thames which they suggest. The selection of the several * normal * appearances from out of the jungle of our optical experiences, to serve as the real sights of which we shall think, is psychologically a parallel phenomenon to the habit of thinking in words, and has a like use. Both are substitutions of terms few and fixed for terms manifold and vague.

This service of sensations as mere signs, to be ignored when they have evoked the other sensations which are their significates, was noticed first by Berkeley and remarked in many passages, as the following : "Signs, being little considered in themselves, or for their own sake, but only in their relative capacity and for the sake of those things whereof they are signs, it comes to pass that the mind overlooks them, so as to carry its attention immediately on to the things signified . . . which in truth and strictness are not seeii, but only suggested and apprehended by means of the proper objects of sight which alone are seen." (Divine Visual Language, § 13.)

Berkeley of course erred in supposing that the thing suggested was not even originally an object of sight, as the sign now is which calls it up. Reid expressed Berkeley's principle in yet clearer language : " The visible appearances of objects are intended by nature only as signs or indications, and the mind passes instantly to the things sigliified, without making the least reflection upon the sign, or even perceiving that there is any such thing. . . . The mind has acquired a confirmed and inveterate habit of inattention to them (the signs). For they no sooner appear than, quick as lightning, the thing signified succeeds and engrosses all our regard. They have no name in language ; and although we are conscious of them when they pass through the mind, yet their passage is so quick and so familiar that it is absolutely unheeded ; nor do they leave any footsteps of themselves, either in tho memory or imagination." (Inquiry, chap. v. §§ 2, 3.)

If we revievr the facts we shall find every grade of nonattention betw^een the extreme form of overlooking mentioned by Eeid (or forms even more extreme still) and complete conscious perception of the sensation present. Sometimes it is literally impossible to become aware of the latter. Sometimes a little artifice or eifort easily leads us to discern it together, or in alternation, with the * object ' it reveals. Sometimes the present sensation is held to he the object or to reproduce its features in undistorted shape, and then, of course, it receives the mind's full glare.

The deepest inattention is to subjective optical sensations, strictly so called, or those which are not signs of outer objects at all. Helmholtz's treatment of these phenomena, muscce volitantes, negative after-images, double images, etc., is very satisfactory. He says : " We only attend with any ease and exactness to our sensations in so far forth as they can be utilized for the knowledge of outward things ; and we are accustomed to neglect all those portions of them which have no significance as regards the external world. So much is this the case that for the most part special artifices and practice are required for the observation of these latter more subjective feelings. Although it might seem that nothing should be easier than to be conscious of one's own sensations, experience nevertheless shows that often enough either a special talent like that showed in eminent degree by Purkinje, or accident or theoretic speculation, are necessary conditions for the discovery of subjective phenomena. Thus, for example, the blind spot on the retina was discovered by Mariotte by the theoretic way ; similarly by me the existence of ' summation '-tones in acoustics. In the majority of cases accident is what first led observers whose attention was especially exercised on subjective phenomena to discover this one or that ; only where the subjective appearances are so intense that they interfere with the perception of objects are they noticed by all men alike. But if they have once been discovered it is for the most part easy for subsequent observers who place themselves in proper conditions and bend their attention in the right direction to perceive them. But in

many cases — for example, in the phenomena of the blind spot, in the discrimination of over-tones and combination-tones from the groundtone of musical sounds, etc. — such a strain of the attention is required, even with appropriate instrumental aids, that most persons fail. The very after-images of bright objects are by most men perceived only under exceptionally favorable conditions, and it takes steady practice to see the fainter images of this kind. It is a commonly recurring experience that persons smitten with some eye-disease which impairs vision suddenly remark for the first time the muscce volitantes which all through life their vitreous humor has contained, but which they now firmly believe to have arisen since their malady ; the truth being that the latter has only made them more observant of all their visual sensations. There are also cases where one eye has gradually grown blind, and the patient lived for an indefinite time witLout knowing it, until, through the accidental closure of the healthy eye alone, the blindness of the other was brought to attention.

' ' Most people, when first made aware of binocular double images, are uncommonly astonished that they should never have noticed them before, although all through their life they had been in the habit of seeing singly only those few objects which were about equally distant with the point of fixation, and the rest, those nearer and farther, which constitute the great majority, had always been double. "We must then learn to turn our attention to our particular sensations, and we learn this commonly only for such sensations as are means of cognition of the outer world. Only so far as they serve this end have our sensations any importance for us in ordinary life. Subjective feelings are mostly interesting only to scientific investigators ; were they remarked in the ordinary use of the senses, they could only cause disturbance. Whilst, therefore, we reach an extraordinary degree of firmness and security in objective observation, we not only do not reach this where subjective phenomena are concerned, but we actually attain in a high degree the faculty of overlooking these altogether, and keeping ourselves independent of their influence in judging of objects, even in cases where their strength might lead them easily to attract our attention." (Physiol. Optik, pp. 431-2.)

Even where the sensation is not merely subjective, as in the cases of which Helmholtz speaks, but is a sign of something outward, we are also liable, as Reid says, to overlook its intrinsic quality and attend exclusively to the image of the * thing ' it suggests. But here everyone can easily notice the sensation itself if he will. Usually we see a sheet of paper as uniformly white, although a part of it may be in shadow. But we can in an instant, if we please, notice the shadow as local color, A man walking towards us does not usually seem to alter his size ; but we can, by setting

our attention in a peculiar way make him appear to do so. The whole education of the artist consists in his learning to see the presented signs as well as the represented things. No matter what the field of view means, he sees it also as it feels — that is, as a collection of patches of color bounded by lines — the whole forming an optical diagram of whose intrinsic proportions one who is not an artist has hardly a conscious inkling. The ordinary man's attention passes over them to their import ; the artist's turns back and dwells upon them for their own sake. * Don't draw the thing as it is, but as it looks ! ' is the endless advice of every teacher to his pupil ; forgetting that what it * is ' is what it would also * look,' provided it were placed in what we have called the * normal ' situation for vision. In this situation the sensation as * sign ' and the sensation as * object ' coalesce into one, and there is no contrast between them.

But a great difficulty has been made of certain peculiar cases which we must now turn to consider. They are cases in which a present sensation, whose existence is supposed to be proved by its outward conditions being there, seems absolutely suppressed or changed by the image of the * thing ' it suggests. This matter carries us back to what was said on p. 218. The passage there quoted from Helmholtz refers to these cases. Ke thinks they conclusively disprove the original and intrinsic spatiality of any of our retinal sensations ; for if such a one, actually present, had an immanent and essential space-determination of its own, that might well be added to and overlaid or even momentarily eclipsed by suggestions of its signification, but how could it possibly be altered or completely suppressed thereby ? Of actually present sensations, he says, being suppressed by suggestions of experience —

" We have not a single well-attested example. In all those illusions Which are provoked by sensations in the absence of their usually exciting objects, the mistake never vanishes by the better understanding of the object really present, and by insight into the cause of deception. Phosphenes provoked by pressure on the eyeball, by traction on the entrance of the optic nerve, after-images, etc. , remain projected into their apparent place in the field of vision, just as the image projected from

a mirror's surface continues to be seen behind the mirror, although wo know that to all these appearances no outward reality corresponds. True enough, we can remove our attention, and keep it removed, from sensations that have no reference to the outer world, those, e.g., of the weaker after-images, and of entoptic objects, etc. . . . But what would become of our perceptions at all if we had the power not only of ignoring, but of transforming into their opposites^ any part of them that differed from that outward experience, the image of which, as that of a present reality, accompanies them in the mind ? " *

'*0n the analogy of all other experience, we should expect that the conquered feelings would persist to our perception, even if only in the shape of recognized illusions. But this is not the case. One does not see how the assumption of originally spatial sensations can explain our optical cognitions, when in the last resort those who believe in these very sensations find themselves obliged to assume that they are overcome by our better judgment, based on experience."

These words, coming from such a quarter, necessarily carry great weight. But the authority even of a Helmholtz ought not to shake one's critical composure. And the moment one abandons abstract generalities and comes to close quarters with the particulars, I think one easily sees that no such conclusions as those we have quoted follow from the latter. But profitably to conduct the discussion we must divide the alleged instances into groups. (a) With Helmholtz, color-perception is equally with spaceperception an intellectual affair. The so-called simultaneous color-contrast, by which one color modifies another alongside of which it is said, is explained by him as an unconscious inference. In Chapter XYII we discussed the color-contrast problem ; the principles which applied to its solution will prove also applicable to part of the present problem. In my opinion, Hering has definitively proved that, when one color is laid beside another, it modifies the sensation of the latter, not by virtue of any mere mental suggestion, aa Helmholtz would have it, but by actually exciting a new nerve -process, to which the modified feeling of color immediately corresponds. The explanation is physiological, not psychological. The transformation of

the original color by the inducing color is due to the disappearance of the physiological conditions under which the first color was produced, and to the induction, under the new conditions, of a genuine new sensation, with which the * suggestions of experience ' have naught to do. That processes in the visual apparatus propagate themselves laterally, if one may so express it, is also shown by the phenomena of contrast which occur after looTdng upon motions of various kinds. Here are a few examples. If, over the rail of a moving vessel, we look at the water rushing along the side, and then transfer our gaze to the deck, a band of planks will appear to us, moving in the opposite