CHAPTER IX.

SYNTHESIS AND CORRELATION.

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N the foregoing chapter we submitted impressions to analysis, and we found that the ultimate psychological results of that analysis were sensations. We saw that these included sensations of the special senses, sensations of pain and general sensibility, and motor-sensations, any or all of which may carry with them an emotional tone pleasureable or the reverse. And, furthermore, we saw that the sensations might in physiological analysis be further decomposed into impulses, the sensation purple, for example, being due to a combination of such physiological impulses. I have used such phrases as "the combination of physiological impulses to give rise to a sensation" to avoid pedantry and circumlocution. But it may be noted that such phrases are, in strictness, open to a very serious criticism. Physiological impulses are waves of molecular change of some sort, and are physical in their nature; while sensations are elements in consciousness, and psychical in their nature. No combination of physical impulses can give rise, it may be said, to a psychical or conscious condition, for there is no community of nature, and no thinkable continuity, between physical states and consciousness. With this criticism I am heartily in agreement. What I should say, in strictness, is, that the physiological conditions which are the concomitants of sensations are due to the combination of physiological impulses, the concomitants of which are infra-conscious, or do not rise above the threshold of consciousness. But such sentences as this are rather cumbrous! While, therefore, I

heartily agree with the spirit of the criticism, I shall for the sake of brevity and clearness use the less accurate phraseology.

We have now to see how the sensation-elements are combined synthetically to form impressions as we know them; how they enter into correlation with each other ; and how they call up through association representations of similar sensation-elements. It must be remembered that the web of consciousness, even in sense-experience, is wonderfully complex, with a complexity which beggars description. All that we can do in this chapter is to consider some of the more important of the correlations, and to indicate the manner in which sensations enter into synthetic union.

When we look at such an object as a cubical brick lying on a table at a distance of three or four feet from us, we obtain a definite impression. We are, however, using two eyes. On the retina of each eye there is an image of the brick; but the grouped impulses from the two retinas are factors in a single impression. Moreover, the two images are not quite alike, for the two eyes look out on the brick from slightly different positions, and a little experimenting will show that what is seen with the left eye is not quite the saine aspect of the brick as that which is seen with the right eye. But not only do the different groups of impulses from the two eyes conspire to give rise to a single impression, they also give, or aid in giving, to the impression the specific quality of solidity or spatial depth. This may be experimentally shown with the aid of the stereoscope. By means of this instrument two different flat pictures, taken from slightly divergent points of view, are seen at the same time, the one with the left eye, the other with the right. The effect is a wonderfully perfect illusion that the objects have solidity, and that they stand out in space of three dimensions. Even if the pictures are only momentarily

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illuminated by the electric flash, the illusion of solidity is still conveyed. It would seem, then, that the synthetic combination of the impulses generated by two different retinal images, gives to the visual impression its element of solidity or depth. Now the question arises, have we in this synthesis a combination of physiological impulses, or a combination of sensations? In other words, is the dissociation point of the depth element of a visual impression within the conscious region, or is it infra-conscious? For me, if I can trust my powers of introspection, it is infraconscious. Psychologically I cannot analyse the element of solidity in a given impression. I know perfectly well that it results from the synthetic combination of two retinal images. But it is for me a sensation which, though it results from a physiological synthesis, resists all my efforts after psychological dissociation. Others, however, affirm that it is possible by close analysis of visual impressions to distinguish between the two retinal images. For them the dissociation-point is within the conscious region. In any case the important point to notice is that the combination, however and wherever occurring (and it may physiologically occur in the lower brain-centres), does not merely produce a composite, and hence more or less blurred impression, but a clear-cut and definite impression, with a new quality, that of outness, distance, and solidity. If it be asked why the synthetic combination gives rise to this new quality in the psychical product, we must reply that we do not know. We know practically nothing concerning the ultimate "whys" of consciousness, though we know a little concerning the proximate "whys" and the "hows." We do not know why rays of a certain vibration frequency give the sensation red, while rays of another vibration frequency give the sensation green. We do not know why, when there are combined upon the retina the rays which give the sensation red with the rays which give the sensation green, we get a quite new sensation

different from both, which we call yellow. So, for psychology, visual distance and solidity, the third dimensional space element, appears to be an ultimate element. We may explain, or attempt to explain, physiologically how it is generated, but why it takes this form we cannot say.

There are, however, other factors, which aid in giving to visual impressions their quality of distance, and to space, as presented to the eye, its element of depth. If we hold a pencil on our forefinger, about ten inches from our face, and focus our eyes alternately on this and on some distant object, we shall become aware, by the exercise of a little introspective attention, that there are present psychical elements other than those belonging to the visual sensations as such. These elements are sensations associated with certain movements of the eyes, or parts of the eyes, during the process of focussing. Physiology shows that these movements are of two kinds. First, there are movements of greater or less convergence of the two eyes, so as to make the image fall on the most sensitive area of the retina; and secondly, there are movements of accommodation within the eye by which, through a change in the curvature of the hinder surface of the lens, the image is rendered more clearly defined. It is probably impossible psychologically to distinguish between these two, though physiologically they are quite distinct. Moreover, these motor-sensations are seldom rendered focal except for purposes of psychological analysis. They are normally marginal, and are only dragged to the light of focal consciousness to satisfy the demands of scientific analysis. But I have written so far in vain if I have not made it clear that the influence of the subconscious margin, in determining our states of consciousness, is of extreme importance, and needs constant recognition.

When we see, then, a focal object set in a visual field, there are, apart from and in addition to visual sensations (that is to say, sense-data supplied by the retinas), motor

sensations (that is to say, sense-data due to movements of or in the eyes). And these latter, in correlation with the binocular factor, aid in giving to our impressions of sight their visual depth. They aid also in giving to objects their solidity. For when we take in the form or figure of any object, our eyes wander over its several parts with varying convergence and accommodation for distance. And it must be remembered that as, during our waking hours, we look hither and thither, and comprise now this, now that, and now the other object in our glance, we proceed by successive gradations, the motor impulses forming a continuous series associated with constantly changing adjustments. This continuity of the motor series is probably not a little helpful in giving rapidity and exactness to our visual localization.

It appears, therefore, that the depth element in visual impressions, as given through the combined impulses from two retinas, is correlated with motor-sensations of convergence and adjustment. We have now to see that the extension element, as given in the grouped impulses from each retina, is likewise correlated with other motor-sensations. To this end, let us begin by noting what takes place when we observe a moving object in the field of vision. We are watching, we will suppose, a game of billiards. Again and again our eyes follow the balls in their movements across the table. We are at most but dimly subconscious of our own motor-sensations as our eyes follow the movements of the billiard balls. It is the object itself as moving on which in each case attention is fixed. The ball is in the focus of vision, and as it moves the surrounding margin is gradually changing. Often two balls are moving at once. We fix our attention on one of them, and follow it with our eyes; but we are aware of the movements of the other as changes in the margin of vision; and we can leap with our eyes from one ball to the other with almost infallible accuracy. We can, if we like, while white and red are moving, fix our eyes