In the article eye (q.v.), it has been shown (as, indeed, necessarily follows from optical principles) that the eye does actually undergo specific modifications, depending for their amount on the distance of the object; and there therefore seems an intrinsic probability that these distantial variations in the organ of sight are correlated to those facts of our consciousness which we denominate variations of visible (not tangible) dis tance; and as, furthermore, it may be demonstrated by optical experiments that the "apparent distance" of a visible point is directly modified, to our perception, by a simple change in the mutual inclination of its diverging rays, it seems an inevitable conclusion, that that agency of light which suggests to our ;Rinds differences of distance is compe tent to suggest distance itself.
Berkeley was quite aware of the necessary connection which subsists between the distance of an object and the divergency of the rays it emits, though it may be doubted whether he adequately weighed the importance of the train of consequences evoked within the eye itself by this variable divergency of incidence; but he affirms that the mind is not by these means helped to a conception of distance except in so far as by experience we have found that increased divergency, carried to the extent of pro ducing "confused vision," is constantly associated with diminished distance. And in proof that this association is merely accidental, Berkeley cites a curious optical experi ment, which shows that where the incident rays are caused slightly to converge, instead of their suggesting, as one would be led to expect, that the object is at an enormous dis tance, the result is altogether different; viz., at first, when the eye is close to the lens, and vision distinct, the object is seen at its true distance, but afterward, as the eye is gradually withdrawn, and vision becomes continually more "confused," the object appears to be enlarged in all its dimensions, and to approach nearer and nearer, until it vanishes in mere confusion from the view. " This phenomenon," he says, "entirely subverts the opinion of those who will have us judge of distance by lines and angles, on which supposition it is altogether inexplicable. To which it may be replied, that the hypothesis being that the mind judges (mediately) by " the various divergency of rays," it cannot fairly be tested by experimenting with rays that are convergent, and that necessarily produce conditions of vision the reverse of those normally prevalent. But, besides, it is now certain that the explanation given by Berkeley is not the true -me; for it has been shown, by prof. Wheatstone, that when the dimensions of a retinal picture are continuously increased (as is the case in the above experiment), the object appears to approach in the most evident manner.
From the doctrine of Berkeley, that the sight is not immediately perceptive of dis tance, it necessarily follows that the parts of a solid object will not be seen as some of them more remote than others, but as if situated all in one and the same plane. This opinion has accordingly been maintained by more recent writers; yet its unsoundness seems manifest; for, if objects be originally seen, not as solid objects, but as perspec tive representations on a plane, then this plane must be seen either at no distance (which Is absurd). or at the same distance for all objects (for which no reason and no evidence can be assigned); or at distances varying with the distances of the objects; but as the last two and only tenable suppositions assume the visual perception of distance, which is the very principle sought to be invalidated, the theory is thus shown to be futile and aelf•contradietory. If it be admitted that, by the constitution of the organ of sight in relation to light, we are perceptive of distance at all, it is in the highest degree proba ble, judging not only from analogy, but from the proved distantial affections of the eye, that we perceive by the sight degrees of distance; and a perception of the latter implies, it has been shown, a perception a trine] dimensions. Now, although it is strenuously maintained by the adherents of Berkeley that this is not a primitive attri bute of vision, it is not denied by any, that in the exercise of our mature sight, we do undoubtedly perceive the outness, the distance, and the trim] extension of visible objects; but, say they, these very qualities, not being modifications of light or colors, are only in appearance directly perceived by the eye; they are, in fact, the product of tactual experience, but by long and invariable association with the phenomena proper to sight, are now instantaneously suggested by them, in a manner so intimate • that the two sets of perceptions have become, to our consciousness, indissolubly one. This in effect, is to affirm that we cannot see an object to be possessed of trinal dimensions, until its occupancy of space is assured to us by the touch; whereas, we venture to main tain, that we see objects to occupy space, and that what we owe to experience is a knowl edge that the major part of these visible appearances have, underlying them, that which, on our making a proper disposition of our bodies, will produce in us tactual sensations. We advisedly say the ,major part, because there arc many objects in nature, such, for instance, as wreaths of smoke and vapor, which, though to the sight visibly pos sessed of trinel dimensions, are totally imperceptible to the touch. And this suggests Mm remark, that a great diversity of opinion has arisen out of the ambiguous meaning of the word " solidity," by which those who agree with Berkeley always signify, not mere occupancy of space, which, as we have shown, may be associated with a total absence of tangible qualities, but "resistant extension" in three dimensions, which, beyond all doubt, is solely cognizable by the touch. It is, of course, in the former sense alone that we vindicate to the sight an immediate perception of " solidity;" and we do not use the term "occupancy of space" at all in the sense of mechanical exclusion, an idea manifestly derived from touch, but only as affirming the immediately perceived trinal extension of visible objects. It may also be here remarked, that there are many substances of extreme hardness, and therefore in the highest degree perceptible to the touch, which, though set in a strong light, remain quite invisible; as, for example, the sheets of plate-glass used by prof. Pepper in producing his " ghost-illusions." Indeed, strictly speaking, all perfectly transparent substances, and all perfectly reflecting (pol ished) surfaces, are invisible. Again, there are many appearances in nature, into our perception of which there enters no element of tactual experience, even as it respects vaiiety of. surface (color, of course,•being excluded from this consideration); as, for example, the waves of the sea, spread out in long undulating lines, or breaking in foam upon the shore; and all those objects which, by their minuteness and the delicate diversifications of their shape and outline, elude the cognizance of the touch. These
facts are adduced simply to illustrate the complete distinction and independence which subsists between the two sets of sensations, originating, respectively, with the sight and the touch—a point strongly enforced by Berkeley himself, who did not hesitate to affirm, not only that our habit of referring the two sets of sensations to the same objects is the mere effect of our having experienced them together, but' that the two sets of ideas thus "intromitted into the mind" belong, in fact, to two classes of objects, numerically distinct —the one outward, distant, and tangible; the other visible, but at no distance, and there fore, in reality, contained within the mind itself. But, as has been well pointed out by Mr. Bailey, this very distinction, taken in conjunction with the undoubted fact, that we do in maturity apprehend by the sight the distance of visible objects, furnishes a strong pre sumption that those perceptions cannot have belonged originally to the touch.' How little tactual sensations are able to modify visual 'perceptions is exemplified as Mr. Bailey remarks) by the fact, that "a straight stick, with one end placed in a basin of water, would still appear to the sight to be bent at that end, after a thousand proofs by the touch that it was otherwise." In the same way, the finger immersed in water appears " unnaturally bent, though the experimenter feels it to be otherwise." The nature of "visible" or "apparent distance," and how it compares with "real distance," we shall consider when we come to speak of apparent, as contrasted with real magnitude. But we will first inquire what are the optical conditions within the eye itself which determine our perception of the direction of a visible point. Every such point, as we have before remarked, radiates to the eye a cone of diverging rays, whose base is situated in the pupil of the eye; and these rays being refracted in their passage through the eye, are brought to a on the retina, thus forming another cone, the base of which is opposed to that of the incident cone, while its apex coincides with the focal point of the refracted rays. If the point of emission be placed directly before the eye, it will be seen in the direction of the common axis of these two cones; or, in other words, in the optic axis; and the central point in which tins axis passes through the common basis of the two cones is called the optical center of the eye. Let us auppose, now, another visible point, a little above the first, but at an equal distance from the eye; this, too, will send forth to the eye diverging rays, which will, in like manner, be refracted to a focus upon the retina at a point a little below the preceding; and the line of visible direction will pass from the point of convergence on the retina through the optical center. Now, it is evident that the rays, by means of which we see a visible point, come to the retina from all possible directions within the limits of the cones they collectively form. How comes it, then, that we perceive the object only in one deter minate direction? The explanation usually given (founded on experiments in which a portion of the cone has been excluded without apparent change in the visible direction) is, that, by the constitution of the sense. upon any point in the retina receiving the apex of a cone of rays, we perceive the object in a right line extending from that point of the retina through the optical center; or, according to others, in a right line perpendicular to the surface of the retina at that point. It will, however, be more consistent with the principle that the incidence of light is accompanied by a positive action, related to its direction as well as to its other properties, if we express the law of visible direction by saying, that upon a multitude of rays from all possible directions falling upon a retinal point, the perceived direction is the mean or resultant of them all; which is just as true an account of the phenomena, and amounts, we think, to something more than a verbal distinction. The optical facts we have thus, in brief, indicated, if followed out with respect to all the visible points forming the objects of sight, render it evident that inver ted images of the latter are painted upon the retina and the inquiry is prompted, how these can give rise to erect vision. On reflection it will be seen that an explanation of this old and much debated "paradox of vision," is involved in the preceding statement _ of the law of visible direction, in whichever way it may be expressed; and, as has been shown by previous writers, the difficulty itself has arisen solely from the assumption, contrary to fact, that we see the retinal pictures, whereas, considered as innie-es, they are not even the means, but only the concomitants of that operation of light by which we see. Even this important distinction, however, does not convey the whole truth; and we trust it will not be deemed an unnecessary refinement, if we point out that in a strict sense there is no image upon the retina, but only a concourse of rays, which, to the eye of another person, will undoubtedly give the perception of an image, but cannot be affirmed to exist, as ay image, except in relation to this second observer. It is therefore with this reservation that theterm retinal images is here made use of. As a conse quence of all the lines of visible direction passing through the center of the eye, it fol lows that as an object recedes from or approaches the eye, its retinal image 'becomes proportionally less, or greater; and in like manner, the visible object itself varies in magnitude, under certain limitations, to be presently referred to, with every change in its distance. But as " the magnitude of the object which exists without the mind, con tinues always invariably the same," it is evident., argues Berkeley, that "whenever we speak of the magnitude of anything, we must. mean the tangible magnitude," which alone is measurable by settled stated lengths." The sense in which this is true, clearly illustrates the nature of magnitude'and distance, as apprehended by the sight, in contra distinction to what is called real magnitude and real distance, the product of tactual experience. It must not be understood as to the touch of a superiority in mensurative capacity, but as simply meaning, that by the touch we come to know that the external world is endowed with resistant qualities—such as hardness, impenetra bility, and incompressibility—qualities which we cannot conceive as modifiable by our bare visual perception of them; and from this experience accrues our conception of the reality and actuality of the magnitude and distance of objects, accompanied by a belief that the variability of magnitude and distance perceived by the sight is an appearance only, and dependent on conditions solely of a subjective kind. If this be a correct view, we are not forced to deny with Berkeley that the objects of sight are numerically the same as those we are cognizant of by the touch.