Let us trace this correspondence of impressions, sensa- tions, and perceptions as far as we can; beginning with that which is first in order, the impression made upon the bodily organ. — But alas! we know not of what nature these impressions are, far less how they excite sensations in the mind.
We know that one body may act upon another by pres- sure, by percussion, by attraction, by repulsion, and pro- bably in many other ways, which we neither know nor have names to express. But in which of these ways ob- jects, when perceived by us, act upon the organs of sense, these organs upon the nerves, and the nerves upon the brain, we know not. Can any man tell me how, in vision, the rays of light act upon the retina, how the retina acts upon the optic nerve, and how the optic nerve acts upon the brain? Xo man can. When I feel the pain of the gout in my toe, I know that there is some unusual im- pression made upon that part of the body. But of what kind is it? Are the small vessels distended with some re- SECT. XXI.] OF SEEING. 209 dundant elastic or unelastic fluid? Are the fibres un- usually stretched? Are they torn asunder by force, or gnawed.and corroded by some acrid humour? I can an- swer none of these questions. All that 1 feel is pain, which is not an impression upon the body, but upon the mind; and all that I perceive by this sensation is, that some distemper in my toe occasions this pain. But as I know not the natural temper and texture of my toe when it is at ease, I know as little what change or disorder of its parts occasions this uneasy sensation. In like manner, in every other sensation, there is, without doubt, some im- pression made upon the organ of sense; but an impression of which we know not the nature. It is too subtile to be discovered by our senses, and we may make a thousand conjectures without coming near the truth. If we under- stood the structure of our organs of sense so minutely as to discover what effects are produced upon them by external objects, this knowledge would contribute nothing to our perception of the object; for they perceive as distinctly who know least about the manner of perception as the greatest adepts. It is necessary that the impression be made upon our organs, but not that it be known. Nature carries on this part of the process of perception without our consciousness or concurrence.
But we cannot be unconscious of the next step in this process, the sensation of the mind, which always imme- diately follows the impression made upon the body. It is essential to a sensation to be felt, and it can be nothing more than we feel it to be. If we can only acquire the habit of attending to our sensations, we may know them perfectly. But how are the sensations of the mind pro- duced by impressions upon the body? Of this we are ab- solutely ignorant, having no means of knowing how the body acts upon the mind, or the mind upon the body. When we consider the nature and attributes of both, they seem to be so different and so unlike that we can rind no handle by which the one may lay hold of the other. There is a deep and dark gulf between them, which our under- 210 OF THE HUMAN MIND. [CHAP. VI.
standing cannot pass; and the manner of their corre- spondence and intercourse is absolutely unknown.
Experience teaches us, that certain impressions upon the body are constantly followed by certain sensations of the mind; and that, on the other hand, certain determina- tions of the mind are constantly followed by certain mo- tions in the body; but we see not the chain that ties these things together. Who knows but their connexion may be arbitrary, and owing to the will of our Maker? Perhaps the same' sensations might have been connected with other impressions or other bodily organs. Perhaps we might have been so made as to taste with our fingers, to smell with our ea?s, and to hear by the nose. Perhaps we might have been so made as to have all the sensations and perceptions which we have, without any impression made upon our bodily organs at all.
However these things may be, if nature had given us nothing more than impressions made upon the body, and sensations in our minds corresponding to them, we should in that case have been merely sentient, but not percipient beings. We should never have been able to form a con- ception of any external object, far less a belief of its ex- istence. Our sensations have no resemblance to external objects; nor can we discover, by our reason, any neces- sary connexion between the existence of the former and that of the latter.
We might perhaps have been made of such a constitu- tion as to have our present perceptions connected with other sensations. We might perhaps have had the per- ception of external objects, without either impressions upon the organs of sense or sensations. Or, lastly, The per- ceptions we have might have been immediately connected with the impressions upon our organs without any inter- vention of sensations. This last seems really to be the case in one instance, to wit, in our perception of the visible figure of bodies, as was observed in the 8th Section of this Chapter.
The process of nature in perception by the senses may SECT. XXI.] OF SEEING. 211 therefore be conceived as a kind of drama, wherein some things are performed behind the scenes, others are repre- sented to the mind in different scenes, one succeeding another. The impression made by the object upon the organ, either by immediate contact, or by some inter- vening medium, as well as the impression made upon the nerves and brain, is performed behind the scenes, and the mind sees nothing of it. But every such impression, by the laws of the drama, is followed by a sensation, which is the first scene exhibited to the mind; and this scene is quickly succeeded by another, which is the perception of the object.
In this drama nature is the actor, we are the spectators. We know nothing of the machinery by means of which every different impression upon the organ, nerves, and brain, exhibits its corresponding sensation; or of the ma- chinery by means of which each sensation exhibits its cor- responding perception. We are inspired with the sensa- tion, and we are inspired with the corresponding percep- tion, by means unknown. And because the mind passes immediately from the sensation to that conception and belief of the object which we have in perception, in the same manner as it passes from signs to the things signified by them, we have therefore called our sensations signs of external objects; finding no word more proper to express the function which nature hath assigned them in percep- tion, and the relation which they bear to their corre- sponding objects.
There is no necessity of a resemblance between the sign and the thing signified: and indeed no sensation can resemble any external object. But there are two things necessary to our knowing things by means of signs. First, That a real connexion between the sign and the thing sig- nified be established, either by the course of nature or by the will and appointment of men. When they are con- nected by the course of nature, it is a natural sign; when by human appointment, it is an artificial sign. Thus, smoke is a natural sign of fire; certain features are natural si^ns of anger: but our words, whether expressed by articulate 212 OF THE HUMAN MIND. [CHAP. VI.
sounds or by writing, are artificial signs of our thoughts and purposes.
Another requisite to our knowing things by signs is, that the appearance of the sign to the mind be followed by the conception and belief of the thing signified. Without this the sign is not understood or interpreted; and therefore is no sign to us, however fit in its own nature for that purpose.
Now, there are three ways in which the mind passes from the appearance of a natural sign to the conception and belief of the thing signified; by original principles of our constitution, by custom, and by reasoning.
Our original perceptions are got in the first of these ways, our acquired perceptions in the second, and all that reason discovers of the course of nature in the third. In the first of these ways nature, by means of the sensations of touch, informs us of the hardness and softness of bodies; of their extension, figure, and motion; and of that space in which they move and are placed, as hath been already explained in the fifth Chapter of this inquiry. And in the second of these ways she informs us, by means of our eyes, of almost all the same tilings which originally we could perceive only by touch.
In order, therefore, to understand more particularly how we learn to perceive so many things by the eye, which originally could be perceived only by touch, it will be proper, first, To point out the signs by which those things are exhibited to the eye, and their connexion with the things signified by them; and, secondly, To consider how the experience of this connexion produces that habit by which the mind, without any reasoning or reflection, passes from the sign to the conception and belief of the thing signified.
Of all the acquired perceptions which we have by sight, the most remarkable is the perception of the distance of objects from the eye; we shall therefore particularly con- sider the signs by which this perception is exhibited, and only make some general remarks with regard to the signs which are used in other acquired perceptions.
OF SEEING. 213 SECT. XXII.
Df the Signs by which we learn to perceive Distance from the Eye.
IT was before observed in general, That the original per- ceptions of sight are signs which serve to introduce those that are acquired: but this is not to be understood as if no other signs were employed for that purpose. There are several motions of the eyes, which, in order to distinct vision, must be varied, according as the object is more or less distant; and such motions being by habit connected with the corresponding distances of the object, become signs of those distances. These motions were at first vo- luntary and unconfined; but as the intention of nature was to produce perfect and distinct vision by their means, we soon learn by experience to regulate them according to that intention only, without the least reflection.
A ship requires a different trim for every variation of the direction and strength of the wind: and, if we may be allowed to borrow that word, the eyes require a different trim for every degree of light, and for every variation of the distance of the object while it is within certain limits. The eyes are trimmed for a particular object, by contracting certain muscles, and relaxing others; as the ship is trim- med for a particular wind, by drawing certain ropes and slackening others. The sailor learns the trim of his ship, as we learn the trim of our eyes, by experience. A ship, although the noblest machine that human art can boast, is far inferior to the eye in this respect, that it requires art and ingenuity to navigate her; and a sailor must know what ropes he must pull and what he must slacken to fit her to a particular wind: but with such superior wisdom is the fabric of the eye and the principles of its motion contrived, that it requires no art or ingenuity to see by it. Even that part of vision which is got by experience is at- tained by idiots. We need not know what muscles we are to contract, and what we are to relax, in order to fit the eye to a particular distance of the object.
214 OF THE HUMAN MIND. [CHAP. VI.
But although we are not conscious of the motions we perform in order to fit the eyes to the distance of the ob- ject, we are conscious of the effort employed in producing these motions; and probably have some sensation which accompanies them, to which we give as little attention as to other sensations. And thus an effort consciously ex- erted, or a sensation consequent upon that effort, comes to be conjoined with the distance of the object which gave occasion to it, and by this conjunction becomes a sign of that distance. Some instances of this will appear in con- sidering the means or signs by which we learn to see the distance of objects from the eye. In the enumeration of these we agree with Dr. Porterfield, notwithstanding that distance from the eye, in his opinion, is perceived origi- nally, but, in our opinion, by experience only.
In general, when a near object affects the eye in one manner, and the same object, placed at a greater distance, affects it in a different manner; these various affections of the eye become signs of the corresponding distances. The means of perceiving distance by the eye will therefore be explained, by showing, in what various ways objects af- fect the eye differently, according to their proximity or distance.
1. It is well known, that to see objects distinctly at various distances, the form of the eye must undergo some change. And nature hath given us the power of adapting it to near objects, by the contraction of certain muscles, and to distant objects by the contraction of other muscles. As to the manner in which this is done, and the muscular parts employed, anatomists do not altogether agree. The ingenious Dr. Jurin, in his excellent essay on distinct and indistinct vision, seems to have given the most probable account of this matter; and to him I refer the reader.
But whatever be the manner in which this change of the form of the eye is effected, it is certain that young people have commonly the power of adapting their eyes to all the distances of the object, from six or seven inches to fifteen or sixteen feet; so as to have perfect and distinct vision at any distance within these limits. From this it follows, that the effect we consciously employ to adapt the eye to SECT. XXII.] OF SEEING. 215 any particular distance of objects within these limits, will be connected and associated with that distance, and will become a sign of it. When the object is removed beyond the farthest limit of distinct vision it will be seen indis- tinctly; but more or less so according as its distance is greater or less: so that the degrees of indistinctness of the object may become the signs of distances considerably be- yond the farthest limit of distinct vision.
If we had no other mean but this of perceiving distance of visible objects, the most distant would not appear to be above twenty or thirty feet from the eye, and the tops of houses and trees would seem to touch the clouds; for in that case the signs of all greater distances being the same, they have the same signification, and give the same per- ception of distance.
But it is of more importance to observe, that because the nearest limit of distinct vision in the time of youth, when we learn to perceive distance by the eye, is about six or seven inches, no object seen distinctly ever appears to be nearer than six or seven inches from the eye. We can by art make a small object appear distinct, when it is in reality not above half an inch from the eye; either by using a single microscope or by looking through a small pin-hole in a card. When by either of these means an object is made to appear distinct, however small its dis- tance is in reality, it seems to be removed at least to the distance of six or seven inches, that is, within the limits of distinct vision.
This observation is the more important because it af- fords the only reason we can give why an object is mag- nified either by a single microscope, or by being seen through a pin-hole; and the only mean by which we can ascertain the degree in which the object will be magnified by either. Thus, if the object is really half an inch dis- tant from the eye, and appears to be seven inches distant, its diameter will seem to be enlarged in the same propor- tion as its distance, that is, fourteen times.
2. In order to direct both eyes to an object the optic axes must have a greater or less inclination, according as 216 OF THE HUMAN MIND. [CHAP. VI, the object is nearer or more distant. And although we are not conscious of this inclination, yet we are conscious of the effort employed in it. By this mean we perceive small distances more accurately than we could do by the conformation of the eye only. And therefore, we find, that those who have lost the sight of one eye are apt, even within arm's length, to make mistakes in the distance of objects, which are easily avoided by those who see with both eyes. Such mistakes are often discovered in snuffina a candle, in threading a needle, or in filling a teacup.
When a picture is seen with both eyes and at no great distance, the representation appears not so natural as when it is seen only with one. The intention of painting being to deceive the eye and to make things appear at different distances, which in reality are upon the same piece of canvass, this deception is not so easily put upon both eyt - as upon one; because we perceive the distance of visible objects more exactly and determinately with two eyes than with one. If the shading and relief be executed in the best manner, the picture may have almost the same appearance to one eye as the objects themselves would have, but it cannot have the same appearance to both. This is not the fault of the artist, but an unavoidable im- perfection in the art. And it is owing to what we just now observed, that the perception we have of the distance of objects by one eye is more uncertain and more liable to deception than that which we have by both.
The great impediment, and I think the only invincible impediment, to that agreeable deception of the eye which the painter aims at, is the perception which we have of the distance of visible objects from the eye, partly by means of the conformation of the eye, but chiefly by means of the inclination of the optic axes. If this perception could be removed, I see no reason why a picture might not be made so perfect as to deceive the eye in reality and to be mistaken for the original object. Therefore, in order to judge of the merit of a picture, we ought, as much as possible, to exclude these two means of perceiving tin distance of the several parts of it.
SECT. XXII.] OF SEEING. 217 In order to remove this perception of distance, the con- noisseurs in painting use a method which is very proper. They look at the picture with one eye, through a tube which excludes the view of all other objects. By this method, the principal mean whereby we perceive the dis- tance of the object, to wit, the inclination of the optic axes is entirely excluded. I would humbly propose, as an im- provement of this method of viewing pictures, that the aperture of the tube next to the eye should be very small. If it is as small as a pin-hole, BO much the better, pro- viding there may be light enough to see the picture clearly. The reason of this proposal is, that when we look at an object through a small aperture, it will be seen distinctly, whether the conformation of the eye be adapted to its dis- tance or not, and we have no means left to judge of the distance, but the light and colouring, which are in the painter's power. If, therefore, the artist performs his part properly, the picture will by this method affect the eye in the same manner that the object represented would do; which is the perfection of this art.
Although this second mean of perceiving the distance of visible objects be more determinate and exact than the first, yet it hath its limits, beyond which it can be of no use. For when the optic axes directed to an object are so nearly parallel that, in directing them to an object yet more distant, we are not conscious of any new effort, nor have any different sensation; there our perception of distance stops; and as all more distant objects affect the eye in the same manner, we perceive them to be at the same distance. This is the reason why the sun, moon, planets, and fixed stars, when seen not near the horizon, appear to be all at the same distance, as if they touched the concave surfacn of a great sphere. The surface of this celestial sphere is at that distance beyond which all objects affect the eye in the same manner. Why this celestial vault appears more distant towards the horizon than towards the zenith will afterwards appear.
3. The colours of objects, according as they are more distant, become more faint and languid, and are tinged L 218 OF THE HUMAN MIND. [CHAP. VI.
more with the azure of the intervening atmosphere: to this we may add, that their minute parts become more indis- tinct, and their outline less accurately defined. It is by these means chiefly that painters can represent objects at very different distances upon the same canvass. And the diminution of the magnitude of an object would not have the effect of making it appear to be at a great distance without this degradation of colour, and indistinctness of the outline and of the minute parts. If a painter should make a human figure ten times less than other human figures that are in the same piece, having the colours as bright, and the outline and minute parts as accurately de- fined, it would not have the appearance of a man at a great distance, but of a pigmy or Lilliputian.
When an object hath a known variety of colours, its distance is more clearly indicated by the gradual dilution of the colours into one another than when it is of one uni- form colour. In the steeple which stands before me at a small distance, the joinings of the stones are clearly per- ceptible; the gray colour of the stone and the white cement are distinctly limited: when I see it at a greater distance, the joinings of the stones are less distinct, and the colours of the stone and of the cement begin to dilute into one another; at a distance still greater the joinings disappear altogether, and the variety of colour vanishes.
In an apple tree, which stands at the distance of about twelve feet, covered with flowers, I can perceive the figure and the colour of the leaves and petals; pieces of branches, some larger, others smaller, peeping through the interval of the leaves, some of them enlightened by the sun's rays, others shaded; and some openings of the sky are perceived through the whole. When I gradually remove from this tree, the appearance, even as to colour, changes every minute. First, the smaller parts and the larger are gra- dually confounded and mixed. The colours of leaves, petals, branches, and sky, are gradually diluted into each other, and the colour of the whole becomes more and more uniform. This change of appearance, corresponding to the several distances, marks the distance more exactly than if the whole object had been of one colour.
SECT. XXII.] OF SEEING. 219 Dr. Smith, in his Optics, gives us a very curious ob- servation made by Bishop Berkeley in his travels through Italy and Sicily. He observed, That, in those countries, cities and palaces seen at a great distance appeared nearer to him by several miles than they really Avere: and he veiy 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 has been assigned as the reason why the Italian painters com- monly give a more lively colour to the sky than the Flemish. Ought they not, for the same reason, to give less degradation of 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. Walk- ing by the seaside in a thick fog, I see an object which seems to me to be a man on horseback, and at the dis- tance of about half a mile. My companion, who has bet- ter eyes, or is more accustomed to see such objects in such circumstances, assures me, that it is a seagull, and not a man on horseback. Upon a second view, I imme- diately assent to his opinion; and now it appears to me to be a seagull, and at the distance only of seventy or eighty yards. The mistake made on this occasion, and the cor- rection 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 conclusion in both cases by habit, and not by ratiocination. And the process of the mind seems to have been this: First, not knowing or not minding the effect of a foggy air on the visible appearance of objects, L2 L2 220 OF THE HUMAN MIND. [CHAP. VI.
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 a man on horseback. Thus the deception is brought about. But when I am assured that it is a seagull, the real magnitude of a seagull, toge- ther with the visible magnitude presented to the eye, im- mediately 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 cause: and now the whole chain of signs, and things sig- nified, seems stronger and better connected than it was before; the half mile vanishes to eighty yards; the man on horseback dwindles to a seagull; I get a new percep- tion, and wTonder how I got the former, or what is be- come 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 accus- tomed to observe, and of which we are aware. Bishop Berkeley therefore committed a mistake, when he attri- buted 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 accus- tomed 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.
SECT. XXII.] OF SEEING. 221 4. We frequently perceive the distance of objects by means of intervening or contiguous objects, whose dis- tance or magnitude is otherwise known. When I per- ceive 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 particular in- formation of the dimensions of such fields or tracts, yet their similitude to others which we know, suggests their dimensions.