agreeable or disagreeable taste.
_Of the Sense of_ SMELLING.
EVERY smell or odour is naturally felt as in the nostrils; not as pressing upon or resisting the organ, not as in any respect external to, or independent of, the organ, but as altogether in the organ, and nowhere else but in the organ, or in the principle of perception which feels in that organ. We soon learn from experience, however, that this sensation is commonly excited by some external body; by a flower, for example, of which the absence removes, and the presence brings back, the sensation. This external body we consider as the cause of this sensation, and we denominate by the same words both the sensation and the power by which the external body produces this sensation. But when we say that the smell is in the flower, we do not thereby mean that the flower itself has any feeling of the sensation which we feel; but that it has the power of exciting this sensation in our nostrils, or in the principle of perception which feels in our nostrils. Though this sensation, and the power by which it is excited, are thus denoted by the same word, this ambiguity of language misleads, in this case, the natural judgments of mankind as little as in the two preceding.
_Of the Sense of_ HEARING.
EVERY sound is naturally felt as in the Ear, the organ of Hearing.
Sound is not naturally felt as resisting or pressing upon the organ, or as in any respect external to, or independent of, the organ. We naturally feel it as an affection of our Ear, as something which is altogether in our Ear, and nowhere but in our Ear, or in the principle of perception which feels in our Ear. We soon learn from experience, indeed, that the sensation is frequently excited by bodies at a considerable distance from us; often at a much greater distance, than those ever are which excite the sensation of Smelling. We learn too from experience that this sound or sensation in our Ears receives different modifications, according to the distance and direction of the body which originally causes it. The sensation is stronger, the sound is louder, when that body is near. The sensation is weaker, the sound is lower, when that body is at a distance. The sound, or sensation, too undergoes some {446} variation according as the body is placed on the right hand or on the left, before or behind us. In common language we frequently say, that the sound seems to come from a great or from a small distance, from the right hand or from the left, from before or from behind us. We frequently say too that we hear a sound at a great or small distance, on our right hand or on our left.
The real sound, however, the sensation in our ear, can never be heard or felt any where but in our ear, it can never change its place, it is incapable of motion, and can come, therefore, neither from the right nor from the left, neither from before nor from behind us. The Ear can feel or hear nowhere but where it is, and cannot stretch out its powers of perception, either to a great or to a small distance, either to the right or to the left. By all such phrases we in reality mean nothing but to express our opinion concerning either the distance or the direction of the body which excites the sensation of sound. When we say that the sound is in the bell, we do not mean that the bell hears its own sound, or that any thing like our sensation is in the bell, but that it possesses the power of exciting that sensation in our organ of Hearing. Though in this, as well as in some other cases, we express by the same word, both the Sensation, and the Power of exciting that Sensation; this ambiguity of language occasions scarce any confusion in the thought, and when the different meanings of the word are properly distinguished, the opinions of the vulgar, and those of the philosopher, though apparently opposite, on examination turn out to be exactly the same.
These four classes of secondary qualities, as philosophers have called them, or to speak more properly, these four classes of Sensations; Heat and Cold, Taste, Smell, and Sound; being felt, not as resisting or pressing upon the organ, but as in the organ, are not naturally perceived as external and independent substances; or even as qualities of such substances; but as mere affections of the organ, and what can exist nowhere but in the organ.
They do not possess, nor can we even conceive them as capable of possessing, any one of the qualities, which we consider as essential to, and inseparable from, external solid and independent substances.
First, They have no extension. They are neither long nor short; they are neither broad nor narrow; they are neither deep nor shallow. The bodies which excite them, the spaces within which they may be perceived, may possess any of those dimensions; but the Sensations themselves can possess none of them. When we say of a Note in Music, that it is long or short, we mean that it is so in point of duration.
In point of extension we cannot even conceive, that it should be either the one or the other.
Secondly, Those Sensations have no figure. They are neither round nor square, though the bodies which excite them, though the spaces within which they may be perceived, may be either the one or the other.
{447} Thirdly, Those Sensations are incapable of motion. The bodies which excite them may be moved to a greater or to a smaller distance.
The Sensations become fainter in the one case, and stronger in the other. Those bodies may change their direction with regard to the organ of Sensation. If the change be considerable, the Sensations undergo some sensible variation in consequence of it. But still we never ascribe motion to the Sensations. Even when the person who feels any of those Sensations, and consequently the organ by which he feels them, changes his situation, we never, even in this case, say, that the Sensation moves, or is moved. It seems to exist always, where alone it is capable of existing, in the organ which feels it. We never even ascribe to those Sensations the attribute of rest; because we never say that any thing is at rest, unless we suppose it capable of motion. We never say that any thing does not change its situation with regard to other things, unless we can suppose it to be capable of changing that situation.
Fourthly, Those Sensations, as they have no extension, so they can have no divisibility. We cannot even conceive that a degree of Heat or Cold, that a Smell, a Taste, or a Sound, should be divided (in the same manner as the solid and extended substance may be divided) into two halves, or into four quarters, or into any number of parts.
But though all these Sensations are equally incapable of division; there are three of them, Taste, Smell, and Sound; which seem capable of a certain composition and decomposition. A skilful cook will, by his taste, perhaps, sometimes distinguish the different ingredients, which enter into the composition of a new sauce, and of which the simple tastes make up the compound one of the sauce. A skilful perfumer may, perhaps, sometimes be able to do the same thing with regard to a new scent. In a concert of vocal and instrumental music, an acute and experienced Ear readily distinguishes all the different sounds which strike upon it at the same time, and which may, therefore, be considered as making up one compound sound.
Is it by nature, or by experience, that we learn to distinguish between simple and compound Sensations of this kind? I am disposed to believe that it is altogether by experience; and that naturally all Tastes, Smells, and Sounds, which affect the organ of Sensation at the same time, are felt as simple and uncompounded Sensations. It is altogether by experience, I think, that we learn to observe the different affinities and resemblances which the compound Sensation bears to the different simple ones, which compose it, and to judge that the different causes, which excite those different simple Sensations, enter into the composition of that cause which excites the compounded one.
It is sufficiently evident that this composition and decomposition is altogether different from that union and separation of parts, which constitutes the divisibility of solid extension.
{448} The Sensations of Heat and Cold seem incapable even of this species of composition and decomposition. The Sensations of Heat and Cold may be stronger at one time and weaker at another. They may differ in degree, but they cannot differ in kind. The Sensations of Taste, Smell, and Sound, frequently differ, not only in degree, but in kind. They are not only stronger and weaker, but some Tastes are sweet and some bitter; some Smells are agreeable, and some offensive; some Sounds are acute, and some grave; and each of these different kinds or qualities, too, is capable of an immense variety of modifications. It is the combination of such simple Sensations, as differ not only in degree but in kind, which constitutes the compounded Sensation.
These four classes of Sensations, therefore, having none of the qualities which are essential to, and inseparable from, the solid, external, and independent substances which excite them, cannot be qualities or modifications of those substances. In reality we do not naturally consider them as such; though in the way in which we express ourselves on the subject, there is frequently a good deal of ambiguity and confusion. When the different meanings of words, however, are fairly distinguished, these Sensations are, even by the most ignorant and illiterate, understood to be, not the qualities, but merely the effects of the solid, external, and independent substances upon the sensible and living organ, or upon the principle of perception which feels in that organ.
Philosophers, however, have not in general supposed that those exciting bodies produce those Sensations immediately, but by the intervention of one, two, or more intermediate causes.
In the Sensation of Taste, for example, though the exciting body presses upon the organ of Sensation, this pressure is not supposed to be the immediate cause of the Sensation of Taste. Certain juices of the exciting body are supposed to enter the pores of the palate, and to excite, in the irritable and sensible fibres of that organ, certain motions or vibrations, which produce there the Sensation of Taste. But how those juices should excite such motions, or how such motions should produce, either in the organ, or in the principle of perception which feels in the organ, the Sensation of Taste; or a Sensation, which not only does not bear the smallest resemblance to any motion, but which itself seems incapable of all motion, no philosopher has yet attempted, nor probably ever will attempt, to explain to us.
The Sensations of Heat and Cold, of Smell and Sound, are frequently excited by bodies at a distance, sometimes at a great distance, from the organ which feels them. But it is a very ancient and well-established axiom in metaphysics, that nothing can act where it is not; and this axiom, it must, I think, be acknowledged, is at least perfectly agreeable to our natural and usual habits of thinking.
The Sun, the great source of both Heat and Light, is at an immense {449} distance from us. His rays, however (traversing, with inconceivable rapidity, the immensity of the intervening regions), as they convey the Sensation of Light to our eyes, so they convey that of Heat to all the sensible parts of our body. They even convey the power of exciting that Sensation to all the other bodies that surround us.
They warm the earth and air, we say; that is, they convey to the earth and the air the power of exciting that Sensation in our bodies. A common fire produces, in the same manner, all the same effects; though the sphere of its action is confined within much narrower limits.
The odoriferous body, which is generally too at some distance from us, is supposed to act upon our organs by means of certain small particles of matter, called Effluvia, which being sent forth in all possible directions, and drawn into our nostrils by the inspiration of breathing, produce there the Sensation of Smell. The minuteness of those small particles of matter, however, must surpass all human comprehension. Inclose in a gold box, for a few hours, a small quantity of musk. Take out the musk, and clean the box with soap and water as carefully as it is possible. Nothing can be supposed to remain in the box, but such effluvia as, having penetrated into its interior pores, may have escaped the effects of this cleansing. The box, however, will retain the smell of musk for many, I do not know for how many years; and these effluvia, how minute soever we may suppose them, must have had the powers of subdividing themselves, and of emitting other effluvia of the same kind, continually, and without any interruption, during so long a period. The nicest balance, however, which human art has ever been able to invent, will not show the smallest increase of weight in the gold box immediately after it has been thus carefully cleaned.
The Sensation of Sound is frequently felt at a much greater distance from the sounding, than that of Smell ever is from the odoriferous body. The vibrations of the sounding body, however, are supposed to produce certain correspondent vibrations and pulses in the surrounding atmosphere, which being propagated in all directions, reach our organ of Hearing, and produce there the Sensation of Sound. There are not many philosophical doctrines, perhaps, established upon a more probable foundation, than that of the propagation of Sound by means of the pulses or vibrations of the air. The experiment of the bell, which, in an exhausted receiver, produces no sensible Sound, would alone render this doctrine somewhat more than probable. But this great probability is still further confirmed by the computations of Sir Isaac Newton, who has shown that, what is called the velocity of Sound, or the time which passes between the commencement of the action of the sounding body, and that of the Sensation in our ear, is perfectly suitable to the velocity with which the pulses and vibrations of an elastic fluid of the same density with the air, are naturally propagated. Dr. {450} Benjamin Franklin has made objections to this doctrine, but, I think, without success.
Such are the intermediate causes by which philosophers have endeavoured to connect the Sensation in our organs, with the distant bodies which excite them. How those intermediate causes, by the different motions and vibrations which they may be supposed to excite on our organs, produce there those different Sensations, none of which bear the smallest resemblance to vibration or motion of any kind, no philosopher has yet attempted to explain to us.
_Of the Sense of_ SEEING.
DR. BERKLEY, in his New Theory of Vision, one of the finest examples of philosophical analysis that is to be found, either in our own, or in any other language, has explained, so very distinctly, the nature of the objects of Sight: their dissimilitude to, as well as their correspondence and connection with those of Touch, that I have scarcely any thing to add to what he has already done. It is only in order to render some things, which I shall have occasion to say hereafter, intelligible to such readers as may not have had an opportunity of studying his book, that I have presumed to treat of the same subject, after so great a master. Whatever I shall say upon it, if not directly borrowed from Dr. Berkley, has at least been suggested by what he has already said.
That the objects of Sight are not perceived as resisting or pressing upon the organ which perceives them, is sufficiently obvious. They cannot therefore suggest, at least in the same manner as the objects of Touch, their externality and independency of existence.
We are apt, however, to imagine that we see objects at a distance from us, and that consequently the externality of their existence is immediately perceived by our sight. But if we consider that the distance of any object from the eye, is a line turned endways to it; and that this line must consequently appear to it, but as one point; we shall be sensible that distance from the eye cannot be the immediate object of Sight, but that all visible objects must naturally be perceived as close upon the organ, or more properly, perhaps, like all other Sensations, as in the organ which perceives them. That the objects of Sight are all painted in the bottom of the eye, upon a membrane called the _retina_, pretty much in the same manner as the like objects are painted in a Camera Obscura, is well known to whoever has the slightest tincture of the science of Optics: and the principle of perception, it is probable, originally perceives them, as existing in that part of the organ, and nowhere but in that part of the organ. No optician, accordingly, no person who has ever bestowed any moderate degree of attention upon the nature of Vision, has ever pretended that distance from the eye was the immediate object of Sight. How it is that, by {451} means of our Sight we learn to judge of such distances Opticians have endeavoured to explain in several different ways. I shall not, however, at present, stop to examine their systems.
The objects of Touch are solidity, and those modifications of solidity which we consider as essential to it, and inseparable from it; solid extension, figure, divisibility, and mobility.
The objects of Sight are colour, and those modifications of colour which, in the same manner, we consider as essential to it, and inseparable from it; coloured extension, figure, divisibility, and mobility. When we open our eyes, the sensible coloured objects, which present themselves to us, must all have a certain extension, or must occupy a certain portion of the visible surface which appears before us. They must too have all a certain figure, or must be bounded by certain visible lines, which mark upon that surface the extent of their respective dimensions. Every sensible portion of this visible or coloured extension must be conceived as divisible, or as separable into two, three, or more parts. Every portion too of this visible or coloured surface must be conceived as moveable, or as capable of changing its situation, and of assuming a different arrangement with regard to the other portions of the same surface.
Colour, the visible, bears no resemblance to solidity, the tangible object. A man born blind, or who has lost his sight so early as to have no remembrance of visible objects, can form no idea or conception of colour. Touch alone can never help him to it. I have heard, indeed, of some persons who had lost their sight after the age of manhood, and who had learned to distinguish by the touch alone, the different colours of cloths or silks, the goods which it happened to be their business to deal in. The powers by which different bodies excite in the organs of Sight the Sensations of different colours, probably depend upon some difference in the nature, configuration, and arrangement of the parts which compose their respective surfaces. This difference may, to a very nice and delicate touch, make some difference in the feeling, sufficient to enable a person, much interested in the case, to make this distinction in some degree, though probably in a very imperfect and inaccurate one. A man born blind might possibly be taught to make the same distinctions. But though he might thus be able to name the different colours, which those different surfaces reflected, though he might thus have some imperfect notion of the remote causes of the Sensations, he could have no better idea of the Sensations themselves, than that other blind man, mentioned by Mr. Locke, had, who said that he imagined the Colour of Scarlet resembled the Sound of a Trumpet. A man born deaf may, in the same manner, be taught to speak articulately. He is taught how to shape and dispose of his organs, so as to pronounce each letter, syllable, and word. But still, though he may have some imperfect idea of the remote causes of {452} the Sounds which he himself utters, of the remote causes of the Sensations which he himself excites in other people; he can have none of those Sounds or Sensations themselves.
If it were possible, in the same manner, that a man could be born without the Sense of Touching, that of Seeing could never alone suggest to him the idea of Solidity, or enable him to form any notion of the external and resisting substance. It is probable, however, not only that no man, but that no animal was ever born without the Sense of Touching, which seems essential to, and inseparable from, the nature of animal life and existence. It is unnecessary, therefore, to throw away any reasoning, or to hazard any conjectures, about what might be the effects of what I look upon as altogether an impossible supposition. The eye when pressed upon by any external and solid substance, feels, no doubt, that pressure and resistance, and suggests to us (in the same manner as every other feeling part of the body) the external and independent existence of that solid substance. But in this case, the eye acts, not as the organ of Sight, but as an organ of Touch; for the eye possesses the Sense of Touching in common with almost all the other parts of the body.
The extension, figure, divisibility, and mobility of Colour, the sole object of Sight, though, on account of their correspondence and connection with the extension, figure, divisibility, and mobility of Solidity, they are called by the same name, yet seem to bear no sort of resemblance to their namesakes. As Colour and Solidity bear no sort of resemblance to one another, so neither can their respective modifications. Dr. Berkley very justly observes, that though we can conceive either a coloured or a solid line to be prolonged indefinitely, yet we cannot conceive the one to be added to the other.
We cannot, even in imagination, conceive an object of Touch to be prolonged into an object of Sight, or an object of Sight into an object of Touch. The objects of Sight and those of Touch constitute two worlds, which, though they have a most important correspondence and connection with one another, bear no sort of resemblance to one another. The tangible world, as well as all the different parts which compose it, has three dimensions, Length, Breadth, and Depth. The visible world, as well as all the different parts which compose it, has only two, Length and Breadth. It presents to us only a plain or surface, which, by certain shades and combinations of Colour, suggests and represents to us (in the same manner as a picture does) certain tangible objects which have no Colour, and which therefore can bear no resemblance to those shades and combinations of Colour. Those shades and combinations suggest those different tangible objects as at different distances, according to certain rules of Perspective, which it is, perhaps, not very easy to say how it is that we learn, whether by some particular instinct, or by some application of either reason or experience, which {453} has become so perfectly habitual to us, that we are scarcely sensible when we make use of it.
The distinctness of this Perspective, the precision and accuracy with which, by means of it, we are capable of judging concerning the distance of different tangible objects, is greater or less, exactly in proportion as this distinctness, as this precision and accuracy, are of more or less importance to us. We can judge of the distance of near objects, of the chairs and tables for example, in the chamber where we are sitting, with the most perfect precision and accuracy; and if in broad daylight we ever stumble over any of them, it must be, not from any error in the Sight, but from some defect in the attention. The precision and accuracy of our judgment concerning such near objects are of the utmost importance to us, and constitute the great advantage which a man who sees has over one who is unfortunately blind. As the distance increases, the distinctness of this Perspective, the precision and accuracy of our judgment gradually diminish. Of the tangible objects which are even at the moderate distance of one, two, or three miles from the eye, we are frequently at a loss to determine which is nearest, and which remotest. It is seldom of much importance to us to judge with precision concerning the situation of the tangible objects which are even at this moderate distance. As the distance increases, our judgments become more and more uncertain; and at a very great distance, such as that of the fixed stars, it becomes altogether uncertain. The most precise knowledge of the relative situation of such objects could be of no other use to the enquirer than to satisfy the most unnecessary curiosity.
The distances at which different men can by Sight distinguish, with some degree of precision, the situation of the tangible objects which the visible ones represent, is very different; and this difference, though it, no doubt, may sometimes depend upon some difference in the original configuration of their eyes, yet seems frequently to arise altogether from the different customs and habits which their respective occupations have led them to contract. Men of letters, who live much in their closets, and have seldom occasion to look at very distant objects, are seldom far-sighted. Mariners, on the contrary, almost always are; those especially who have made many distant voyages, in which they have been the greater part of their time out of sight of land, and have in daylight been constantly looking out towards the horizon for the appearance of some ship, or of some distant shore. It often astonishes a landsman to observe with what precision a sailor can distinguish in the offing, not only the appearance of a ship which is altogether invisible to the landsman, but the number of her masts, the direction of her course, and the rate of her sailing. If she is a ship of his acquaintance, he frequently can tell her name, before the landsman has been able to discover even the appearance of a ship.
{454} Visible objects, Colour, and all its different modifications, are in themselves mere shadows or pictures, which seem to float, as it were, before the organ of Sight. In themselves, and independent of their connection with the tangible objects which they represent, they are of no importance to us, and can essentially neither benefit us nor hurt us. Even while we see them we are seldom thinking of them.
Even when we appear to be looking at them with the greatest earnestness, our whole attention is frequently employed, not upon them, but upon the tangible objects represented by them.
It is because almost our whole attention is employed, not upon the visible and representing, but upon the tangible and represented objects, that in our imaginations we arc apt to ascribe to the former a degree of magnitude which does not belong to them, but which belongs altogether to the latter. If you shut one eye, and hold immediately before the other a small circle of plain glass, of not more than half an inch in diameter, you may see through that circle the most extensive prospects; lawns and woods, and arms of the sea, and distant mountains. You are apt to imagine that the Landscape which is thus presented to you, that the visible Picture which you thus see, is immensely great and extensive. The tangible objects which this visible Picture represents, undoubtedly are so. But the visible Picture which represents them can be no greater than the little visible circle through which you see it. If while you are looking through this circle, you could conceive a fairy hand and a fairy pencil to come between your eye and the glass, that pencil could delineate upon that little glass the outline of all those extensive lawns and woods, and arms of the sea, and distant mountains, in the full and the exact dimensions with which they are really seen by the naked eye.
Every visible object which covers from the eye any other visible object, must appear at least as large as that other visible object. It must occupy at least an equal portion of that visible plain or surface which is at that time presented to the eye. Opticians accordingly tell us, that all the visible objects which are seen under equal angles must to the eye appear equally large. But the visible object, which covers from the eye any other visible object, must necessarily be seen under angles at least equally large as those under which that other object is seen. When I hold up my finger, however, before my eye, it appears to cover the greater part of the visible chamber in which I am sitting. It should therefore appear as large as the greater part of that visible chamber. But because I know that the tangible finger bears but a very small proportion to the greater part of the tangible chamber, I am apt to fancy that the visible finger bears but a like proportion to the greater part of the visible chamber. My judgment corrects my eyesight, and, in my fancy, reduces the visible object, which represents the little tangible one, below its real visible dimensions; and, on the {455} contrary, it augments the visible object which represents the great tangible one a good deal beyond those dimensions. My attention being generally altogether occupied about the tangible and represented, and not at all about the visible and representing objects, my careless fancy bestows upon the latter a proportion which does not in the least belong to them, but which belongs altogether to the former.
It is because the visible object which covers any other visible object must always appear at least as large as that other object, that opticians tell us that the sphere of our vision appears to the eye always equally large; and that when we hold our hand before our eye in such a manner that we see nothing but the inside of the hand, we still see precisely the same number of visible points, the sphere of our vision is still as completely filled, the retina of the eye is as entirely covered with the object which is thus presented to it, as when we survey the most extensive horizon.
A young gentleman who was born with a cataract upon each of his eyes, was, in one thousand seven hundred and twenty-eight, couched by Mr.
Cheselden, and by that means for the first time made to see distinctly. 'At first,' says the operator, 'he could bear but very little sight, and the things he saw he thought extremely large; but upon seeing things larger, those first seen he conceived less, never being able to imagine any lines beyond the bounds he saw; the room he was in, he said, he knew to be but part of the house, yet he could not conceive that the whole house would look bigger.' It was unavoidable that he should at first conceive, that no visible object could be greater, could present to his eye a greater number of visible points, or could more completely fill the comprehension of that organ, than the narrowest sphere of his vision. And when that sphere came to be enlarged, he still could not conceive that the visible objects which it presented could be larger than those which he had first seen. He must probably by this time have been in some degree habituated to the connection between visible and tangible objects, and enabled to conceive that visible object to be small which represented a small tangible object; and that to be great, which represented a great one.
The great objects did not appear to his sight greater than the small ones had done before; but the small ones, which, having filled the whole sphere of his vision, had before appeared as large as possible, being now known to represent much smaller tangible objects, seemed in his conception to grow smaller. He had begun now to employ his attention more about the tangible and represented, than about the visible and representing objects; and he was beginning to ascribe to the latter the proportions and dimensions which properly belonged altogether to the former.
As we frequently ascribe to the objects of Sight a magnitude and proportion which does not really belong to them, but to the objects of {456} Touch which they represent, so we likewise ascribe to them a steadiness of appearance, which as little belongs to them, but which they derive altogether from their connection with the same objects of Touch. The chair which now stands at the farther end of the room, I am apt to imagine, appears to my eye as large as it did when it stood close by me, when it was seen under angles at least four times larger than those under which it is seen at present, and when it must have occupied, at least, sixteen times that portion which it occupies at present, of the visible plain or surface which is now before my eyes.
But as I know that the magnitude of the tangible and represented chair, the principal object of my attention, is the same in both situations, I ascribe to the visible and representing chair (though now reduced to less than the sixteenth part of its former dimensions) a steadiness of appearance, which certainly belongs not in any respect to it, but altogether to the tangible and represented one. As we approach to, or retire from, the tangible object which any visible one represents, the visible object gradually augments in the one case, and diminishes in the other. To speak accurately, it is not the same visible object which we see at different distances, but a succession of visible objects, which, though they all resemble one another, those especially which follow near after one another; yet are all really different and distinct. But as we know that the tangible object which they represent remains always the same, we ascribe to them too a sameness which belongs altogether to it: and we fancy that we see the same tree at a mile, at half a mile, and at a few yards distance. At those different distances, however, the visible objects are so very widely different, that we are sensible of a change in their appearance. But still, as the tangible objects which they represent remain invariably the same, we ascribe a sort of sameness even to them too.
It has been said, that no man ever saw the same visible object twice; and this, though, no doubt, an exaggeration, is, in reality, much less so than at first view it appears to be. Though I am apt to fancy that all the chairs and tables, and other little pieces of furniture in the room where I am sitting, appear to my eye always the same, yet their appearance is in reality continually varying, not only according to every variation in their situation and distance with regard to where I am sitting, but according to every, even the most insensible variation in the altitude of my body, in the movement of my head, or even in that of my eyes. The perspective necessarily varies according to all even the smallest of these variations; and consequently the appearance of the objects which that perspective presents to me. Observe what difficulty a portrait painter finds, in getting the person who sits