PERCEPTION OF SOLIDITY.

389

considers this as still imperfectly explained. I have reason to believe, however, that, having made many experiments with the view of elucidating the point, he inclines to the view that there is a mental effect produced over and above the optical effect, which mental effect overrides the optical impression, and gives a perception really different from the

in those countries, cities and palaces seen at a great distance appeared nearer to him by several miles than they really were; and he very judiciously imputed it to this cause: that the purity of the Italian and Sicilian air gave to very distant objects that degree of brightness and distinctness which, in the grosser air of his own country, was to be seen only in those that are near. The purity of the Italian air hath been assigned as the reason why the Italian painters commonly gave a more lively colour to the sky than the Flemish. Ought they not, for the same reason, to give less degradation to the colours, and less indistinctness of the minute parts, in the representation of very distant objects?

'It is very certain that, as, in air uncommonly pure, we are apt to think visible objects nearer and less than they really are, so, in air uncommonly foggy, we are apt to think them more distant and larger than the truth. Walking by the sea-side in a thick fog, I see an object which seems to me to be a man on horseback, and at the distance of about half a mile. My companion, who has better eyes, or is more accustomed to see such objects in such circumstances, assures me that it is a sea-gull, and not a man on horseback. Upon a second view, I immediately assent to his opinion, and now it appears to me to be a sea-gull, and at the distance only of seventy or eighty yards. The mistake made on this occasion, and the correction of it, are both so sudden, that we are at a loss whether to call them by the name of judgment, or by that of simple perception.

'It is not worth while to dispute about names, but it is evident that my belief, both first and last, was produced rather by signs than by arguments, and that the mind proceeded to the conclusions in both cases by habit, and not by ratiocination. And the process of the mind seems to have been thisFirst, not knowing, or not minding the effect of a foggy air on the visible appearance of objects, the object seems to me to have that degradation of colour, and that indistinctness of the outline, which objects have at the distance of half a mile; therefore, from the visible appearance as a sign, I immediately proceed to the belief that the object is half a mile distant. Then, this distance, together with the visible magnitude, signify to me the real magnitude, which, supposing the distance to be half a mile, must be equal to that of the man on horseback. Thus the deception is brought about. But when I am assured that it is a sea-gull, the real magnitude of a sea-gull, together with the magnitude presented to the eye, immediately suggest the distance, which in this case, cannot be above seventy or eighty yards; the indistinctness of the figure likewise suggests the fogginess of the air as its

literal sensation. The sense of solidity, arising from the conjoined action of two dissimilar views of an object presented to the two eyes, means a suggestion to the mind that one part of the object is farther off than another, as estimated by our locomotive organs; in other words, the impression revives in us an idea of movement to or from the eye in company

cause; and now the whole chain of signs, and things signified, seems stronger and better connected than it was before; the half mile vanishes to eighty yards; the man on horseback dwindles to a sea-gull; I get a new perception, and wonder how I got the former, or what is become of it, for it is now so entirely gone, that I cannot recover it.

'It ought to be observed that, in order to produce such deceptions from the clearness or fogginess of the air, it must be uncommonly clear or uncommonly foggy; for we learn from experience, to make allowance for that variety of constitutions of the air which we have been accustomed to observe, and of which we are aware. Bishop Berkeley therefore committed a mistake, when he attributed the large appearance of the horizontal moon to the faintness of her light, occasioned by its passing through a larger tract of atmosphere; for we are so much accustomed to see the moon in all degrees of faintness and brightness, from the greatest to the least, that we learn to make allowance for it; and do not imagine her magnitude increased by the faintness of her appearance. Besides, it is certain that the horizontal moon seen through a tube which cuts off the view of the interjacent ground, and of all terrestrial objects, loses all that unusual appearance of magnitude.'

The following paragraphs illustrate the effect of intervening objects in aiding our perception of Distance.

'We frequently perceive the distance of objects, by means of intervening or contiguous objects, whose distance or magnitude is likewise known. When I perceive certain fields or tracts of ground to lie between me and an object, it is evident that these may become signs of its distance. And although we have no information of the dimensions of such fields or tracts, yet their similitude to others which we know suggests their dimensions.

'We are so much accustomed to measure with our eye the ground which we travel, and to compare the judgment of distances formed by sight, with our experience or information, that we learn by degrees, in this way, to form a more accurate judgment of the distance of terrestrial objects, than we could do by any of the means before mentioned. An object placed on the top of a high building, appears much less than when placed upon the ground, at the same distance. When it stands upon the ground, the intervening tract of ground serves as a sign of its distance: and the distance, together with the visible magnitude, serves as a sign of its real magnitude. But when the object is placed on high, this sign of its distance is taken away, the remaining signs lead us to place it at a less distance, and this less distance, together with the visible magnitude, becomes a sign of a less real magnitude.'

WHEATSTONE'S THEORY OF BINOCULAR PERCEPTION. 391

with the picture. When the two eyes view the perspective. of a street, there is brought up the idea of a certain amount of walking exertion, or other locomotive measurement, as part of the perception thence arising. The two eyes looking at a footstool bring up in like manner ideas of greater or less remoteness of the parts. Now, the difficulty lies in explaining why two dissimilar pictures projected on the two retina, give rise to the perception of an object in relief.' 'It may be supposed,' says Mr. Wheatstone, 'that we see but one portion of a field of view at the same instant, the one, namely, to which the optic axes are directed, while all other points are seen so indistinctly that the mind does not recognize them to be either single or double, and that the figure is appreciated by directing the point of convergence of the optic axes successively to a sufficient number of its points to enable us to judge accurately of its form.' But observation does not confirm this supposed indistinctness of those parts for which the eyes are not adjusted; on looking at a stereoscopic view, for example, we find that we obtain a clear and distinct picture of the whole, even when the eyes are steadily fixed upon one point, during which act, by the supposition, all points nearer or farther ought to be confusedly and imperfectly perceived. Hence it is that Sir C. Wheatstone has been led to adopt the above-mentioned view of a mental suggestion coming in to present a clear and perfectly formed idea, notwithstanding the optical fact that, for many parts of the view, there actually falls upon the eyes what would be a double and indistinct image. The mind being once accustomed to fully formed views of all kinds, these are revived by the force of association, the main circumstance for determining the view being present; namely, the double aspect which our experience has always connected with a solid effect, or an effect where varying distance is conjoined with lateral extension. This hypothesis appeals to what is undoubtedly a vera causa in the region of mind.

* The subject of Binocular vision has been greatly studied in Germany, and there are at present two opposite opinions as to the manner of

42. Into this matter, however, I do not enter farther than to remark, that the same circumstances that enable us to obtaining a single perception by means of the two dissimilar pictures. Volkmann holds that the unity is arrived at, by the mind disregarding the conflicting parts of the two pictures, and attending only to their points of agreement. To him the dissimilarity is an incumbrance to be shaken off, an obstacle to be surmounted. Wundt, on the other hand, holds that the dissimilarity, far from being an obstruction, is the very instrument or medium of our motion of solidity. It is (after variation of retinal magnitude) the most suggestive of all the optical marks of a third dimension. The more pronounced the dissimilarity, the more emphatic is our sense of solidity and varying distance from the eye.

In the remarks on Double Vision under the Sense of Sight, I have proceeded upon this latter view as best supported by evidence. In the theory of Volkmann, there appears a needless anxiety on the subject of the double picture, as if it would necessarily distract us with two differing representations of one object. It is fancied that each eye sees a complete image in itself, and that the mind must reconcile these two separate images, before attaining the desired unity of perception. But there seems to be a misapprehension in so regarding the question. Each eye does not see the complete picture, but only a part of the picture; the other eye seeing the other part. We might have a body so placed to the two eyes, that the one eye should see one side and the other eye the other side; in which case the double impression is obviously the picture. Experience tells us that an occasion like this -where both eyes must concur to give the whole extent of the picture, or where we see more by the two than by the one-involves a retreating object, or the solid effect. It is no more necessary that the two eyes should give two complete and separate pictures to the mind, than that the two hands embracing the same ball should suggest two balls; or that the thumb and finger grasping a pen should suggest two pens. The eyes are formed to aid and supplement, and not to contradict, each other. In great distances, each eye is sufficient for taking in the view; no addition is made by their conjoint action. This circumstance is to us simply a token of a far prospect. The opposite case where the two pictures have nothing in common is interpreted as the extreme of nearness in the object.

It is in all probability the fact, that one eye takes the lead in vision, the other merely coming in to supply the additions that constitute solid effect; just as in feeling any thing, we use chiefly the right hand (or the left), and attend to its indications, while the other merely corrects or adds to the notion. Our visual ideas would thus be embodied in the sensation of one eye, while the other, making no claim, in the same individual, to have its sensation embodied separately, gives that extension of view and those adjuncts that serve in the full solid effect. Dr. Carpenter has made this remark, with reference to the binocular microscope. The observer uses one eye principally, and for that eye, it is desirable that the instrument should be as perfect as possible; the other eye has no farther use than to bring out the stereoscopic effect.

THE DIRECTION OF AN OBJECT FROM THE EYE. 393

appreciate the distances of different objects, enable us also to appreciate solid effect, or the continuity of an object through varying distances. The definite change in the inclination of the axes, concurring with a definite and proportional change of the retinal magnitude (the tendency to parallelism of the axes accompanying a decrease of retinal magnitude) would suggest the real width of a street to be the same all through; upon which, the diminished picture gives assurance of the increasing remoteness of the successive parts.

A question has been raised as to our mode of perceiving the direction of an object from the eye. On this I would still repeat that direction is not a perception of sight alone; its very meaning precludes the supposition. It implies the locomotive or other movement that would lead us up to the object, or produce a definite change in its appearance. But there is a certain optical effect constantly associated with the sense of direction, as there is with the sense of magnitude or of distance, and this effect it is interesting as a matter of fact to ascertain. Now, it appears most probable that the line of visible direction is a line passing from the place of an object's impression on the retina through the centre of the crystalline lens:* hence we associate an effect on the centre of the retina with a direction in the line of the axis of the eye, while an impression to the right of this point would. suggest a position left of the axis. But without the experience of our moving organs generally, we should never know, either the meaning of direction, or the fact that a certain impression of the retina implied a certain course for us to take in reference to the object. If the optical law had been entirely different; if, for example, an object were to lie in a direction inclined 45° to the plane of its image in the retina, we should equally well become acquainted with direction ; experience would connect the locomotive estimate with the

* This line has been variously stated. Sir David Brewster affirms that it passes through the centre of the eye. See p. 216 of a work entitled, Essai sur les Phosphènes, &c., par le Dr. SERRE, Paris, 1853.