of it, so that they may answer the purposes intended by them. And when we see, in so many other instances, a system of unconnected muscles conspiring so wonderfully in their several functions, without the aid of habit, it needs not be thought strange, that the muscles of the eyes should, without this aid, conspire to give that direction to the eyes, without which they could not answer their end.

We see a like conspiring action in the muscles which contract the pupils of the two eyes; and in those muscles, whatever they be, by which the conformation of the eyes is varied according to the distance of objects.

It ought, however, to be observed, that, although it appears to be by natural instinct that both eyes are always turned the same way, there is still some latitude left for custom.

What we have said of the parallel motion of the eyes, is not to be understood so strictly as if nature directed us to keep their axes always precisely and mathematically parallel to each other. Indeed, although they are always nearly parallel, they hardly ever are exactly so. When we look at an object, the axes of the eyes meet in that object: and, therefore, make an angle, which is always small, but will be greater or less, according as the object is nearer or more remote. Nature hath very wisely left us the power of varying the parallelism of our eyes a little, so that we can direct them to the same point, whether remote or near. This, no doubt, is learned by custom; and accordingly we see, that it is a long time before children get this habit in perfection.

This power of varying the parallelism of the eyes is naturally no more than is sufficient for the purpose intended by it; but by much practice and straining, it may be increased. Accordingly, we see, that some have acquired the power of distorting their eyes into unnatural directions, as others have acquired the power of distorting their bodies into unnatural postures.

Those who have lost the sight of an eye, commonly lose what they had got by custom, in the direction of their eyes, but retain what they had by nature; that is, although their eyes turn and move always together, yet, when they look upon an object, the blind eye will often have a very small deviation from it; which is not perceived by a slight observer, but may be discerned by one accustomed to make exact observations in these matters.

• See the preceding note.

Section XI.

OF OUR SEEING OBJECTS ERECT BY INVERTED IMAGES.

Another phænomenon which hath perplexed philosophers, is, our seeing objects erect, when it is well known that their images or pictures upon the tunica retina of the eye are inverted.

The sagacious Kepler first made the noble discovery, that distinct but inverted pictures of visible objects are formed upon the retina by the rays of light coming from the object. The same great philosopher demonstrated, from the principles of optics, how these pictures are formed-to wit, That the rays coming from any one point of the object, and falling upon the various parts of the pupil, are, by the cornea and crystalline, refracted so as to meet again in one point of the retina, and there paint the colour of that point of the object from which they come. As the rays from different points of the object cross each other before they come to the retina, the picture they form must be inverted; the upper part of the object being painted upon the lower part of the retina, the right side of the object upon the left of the retina, and so of the other parts.*

This philosopher thought that we see objects erect by means of these inverted pictures, for this reason, that, as the rays from different points of the object cross each other before they fall upon the retina, we conclude that the impulse which we feel upon the lower part of the retina comes from above, and that the impulse which we feel upon the higher part comes from below.

Des Cartes afterwards gave the same solution of this phænomenon, and illustrated it by the judgment which we form of the position of objects which we feel with our arms crossed, or with two-sticks that cross each other.

But we cannot acquiesce in this solution. First, Because it supposes our seeing things erect, to be a deduction of reason, drawn from certain premises: whereas it seems to be an immediate perception. And, secondly, Because the premises from which all mankind are supposed to draw this conclusion, never entered into the minds of the far greater part, but are absolutely unknown to them. We have no feeling or perception of the pictures upon the retina, and as little surely

This inverted picture is seen if we take the eye of an ox, for example, and cut away the posterior part of the sclerotica and choroid; but, without this preparation, it is apparent in the eyes of albino animals, of the owl, &c., in which the hard coat and choroid are semi-diaphanous.-H.

of the position of them. In order to see objects erect, according to the principles of Kepler or Des Cartes, we must previously know that the rays of light come from the object to the eye in straight lines; we must know that the rays from different points of the object cross one another before they form the pictures upon the retina; and, lastly, we must know that these pictures are really inverted. Now, although all these things are true, and known to philosophers, yet they are absolutely unknown to the far greatest part of mankind: nor is it possible that they who are absolutely ignorant of them, should reason from them, and build conclusions upon them. Since, therefore, visible objects appear erect to the ignorant as well as to the learned, this cannot be a conclusion drawn from premises which never entered into the minds of the ignorant. We have indeed had occasion to observe many instances of conclusions drawn, either by means of original principles, or by habit, from premises which pass through the mind very quickly, and which are never made the objects of reflection; but surely no man will conceive it possible to draw conclusions from premises which never entered into the mind at

all.

Bishop Berkeley having justly rejected this solution, gives one founded upon his own principles; wherein he is followed by the judicious Dr Smith, in his "Optics;" and this we shall next explain and examine.

That ingenious writer conceives the ideas of sight to be altogether unlike those of touch. And, since the notions we have of an object by these different senses have no similitude, we can learn only by experience how one sense will be affected, by what, in a certain manner, affects the other. Figure, position, and even number, in tangible objects, are ideas of touch; and, although there is no similitude between these and the ideas of sight, yet we learn by experience, that a triangle affects the sight in such a manner, and that a square affects it in such another manner-hence we judge that which affects it in the first manner, to be a triangle, and that which affects it in the second, to be a square. In the same way, finding, from experience, that an object in an erect position affects the eye in one manner, and the same object in an inverted position affects it in another, we learn to judge, by the manner in which the eye is affected, whether the object is erect or inverted. In a word, visible ideas, according to this author, are signs of the tangible; and the mind passeth from the sign to the thing signified, not by means of any similitude between the one and other, nor by any natural principle, but by having found them constantly conjoined in experience, as

the sounds of a language are with the things they signify: so that, if the images upon the retina had been always erect, they would have shewn the objects erect, in the manner as they do now that they are inverted-nay, if the visible idea which we now have from an inverted object, had been associated from the beginning with the erect position of that object, it would have signified an erect position, as readily as it now signifies an inverted one. And, if the visible appearance of two shillings had been found connected from the beginning with the tangible idea of one shilling, that appearance would as naturally and readily have signified the unity of the object as now it signifies its duplicity.

This opinion is, undoubtedly, very ingenious; and, if it is just, serves to resolve not only the phænomenon now under consideration, but likewise that which we shall next consider our seeing objects single with two eyes.

It is evident that, in this solution, it is supposed that we do not originally, and previous to acquired habits, see things either erect or inverted, of one figure or another, single or double; but learn, from experience, to judge of their tangible position, figure, and number, by certain visible signs.

Indeed, it must be acknowledged to be extremely difficult to distinguish the immediate and natural objects of sight, from the conclusions which we have been accustomed from infancy to draw from them. Bishop Berkeley was the first that attempted to distinguish the one from the other, and to trace out the boundary that divides them. And if, in doing so, he hath gone a little to the right hand or to the left, this might be expected in a subject altogether new, and of the greatest subtilty. The nature of vision hath received great light from this distinction; and many phænomena in optics, which before appeared altogether unaccountable, have been clearly and distinctly resolved by it. It is natural, and almost unavoidable, to one who hath made an important discovery in philosophy, to carry it a little beyond its sphere, and to apply it to the resolution of phænomena which do not fall within its province. Even the great Newton, when he had discovered the universal law of gravitation, and observed how many of the phænomena of nature depend upon this, and other laws of attraction and repulsion, could not help expressing his conjecture, that all the phænomena of the material world depend upon attracting and repelling forces in the particles of matter. And I suspect that the ingenious Bishop of Cloyne, having found so many phænomena of vision reducible to the constant association of the ideas of sight

and touch, carried this principle a little beyond its just limits.

In order to judge as well as we can whether it is so, let us suppose such a blind man as Dr Saunderson, having all the knowledge and abilities which a blind man may have, suddenly made to see perfectly. Let us suppose him kept from all opportunities of associating his ideas of sight with those of touch, until the former become a little familiar; and the first surprise, occasioned by objects so new, being abated, he has time to canvass them, and to compare them, in his mind, with the notions which he formerly had by touch; and, in particular, to compare, in his mind, that visible extension which his eyes present, with the extension in length and breadth with which he was before acquainted.

We have endeavoured to prove, that a blind man may form a notion of the visible extension and figure of bodies, from the relation which it bears to their tangible extension and figure. Much more, when this visible extension and figure are presented to his eye, will he be able to compare them with tangible extension and figure, and to perceive that the one has length and breadth as well as the other; that the one may be bounded by lines, either straight or curve, as well as the other. And, therefore, he will perceive that there may be visible as well as tangible circles, triangles, quadrilateral and multilateral figures. And, although the visible figure is coloured, and the tangible is not, they may, notwithstanding, have the same figure; as two objects of touch may have the same figure, although one is hot and the other cold.

We have demonstrated, that the properties of visible figures differ from those of the plain figures which they represent; but it was observed, at the same time, that when the object is so small as to be seen distinctly at one view, and is placed directly before the eye, the difference between the visible and the tangible figure is too small to be perceived by the senses. Thus, it is true, that, of every visible triangle, the three angles are greater than two right angles; whereas, in a plain triangle, the three angles are equal to two right angles; but when the visible triangle is small, its three angles will be so nearly equal to two right angles, that the sense cannot discern the difference. In like manner, the circumferences of unequal visible circles are not, but those of plain circles are, in the ratio of their diameters; yet, in small visible circles, the circumferences are very nearly in the ratio of their diameters; and the diameter bears the same ratio to the circumference, as in a plain circle, very nearly.

Hence it appears, that small visible figures (and such only can be seen distinctly

| at one view) have not only a resemblance to the plain tangible figures which have the name name, but are to all sense the same: so that, if Dr Saunderson had been made to see, and had attentively viewed the figures of the first book of Euclid, he might, by thought and consideration, without touching them, have found out that they were the very figures he was before so well acquainted with by touch.

When plain figures are seen obliquely, their visible figure differs more from the tangible; and the representation which is made to the eye, of solid figures, is still more imperfect; because visible extension hath not three, but two dimensions only. Yet, as it cannot be said that an exact picture of a man hath no resemblance of the man, or that a perspective view of a house hath no resemblance of the house, so it cannot be said, with any propriety, that the visible figure of a man or of a house hath no resemblance of the objects which they represent.

Bishop Berkeley therefore proceeds upon a capital mistake, in supposing that there is no resemblance betwixt the extension, figure, and position which we see, and that which we perceive by touch.

We may further observe, that Bishop Berkeley's system, with regard to material things, must have made him see this question, of the erect appearance of objects, in a very different light from that in which it appears to those who do not adopt his system.

If

In his theory of vision, he seems indeed to allow, that there is an external material world: but he believed that this external world is tangible only, and not visible; and that the visible world, the proper object of sight, is not external, but in the mind. this is supposed, he that affirms that he sees things erect and not inverted, affirms that there is a top and a bottom, a right and a left in the mind. Now, I confess I am not so well acquainted with the topography of the mind, as to be able to affix a meaning to these words when applied to it.

We shall therefore allow, that, if visible objects were not external, but existed only in the mind, they could have no figure, or position, or extension; and that it would be absurd to affirm, that they are seen either erect or inverted, or that there is any resemblance between them and the objects of touch. But when we propose the question, why objects are seen erect and not inverted, we take it for granted, that we are not in Bishop Berkeley's ideal world, but in that world which men who yield to the dictates of common sense, believe themselves to inhabit. We take it for granted, that the objects both of sight and touch, are external, and have a certain figure, and

a certain position with regard to one another, and with regard to our bodies, whether we perceive it or not.

When I hold my walking-cane upright in my hand, and look at it, I take it for granted that I see and handle the same individual object. When I say that I feel it erect, my meaning is, that I feel the head directed from the horizon, and the point directed towards it; and when I say that I see it erect, I mean that I see it with the head directed from the horizon, and the point towards it. I conceive the horizon as a fixed object both of sight and touch, with relation to which, objects are said to be high or low, erect or inverted; and when the question is asked, why I see the object erect, and not inverted, it is the same as if you should ask, why I see it in that position which it really hath, or why the eye shews the real position of objects, and doth not shew them in an inverted position, as they are seen by a common astronomical telescope, or as their pictures are seen upon the retina of an eye when it is dissected.

Section XII.

THE SAME SUBJECT CONTINUED.

It is impossible to give a satisfactory answer to this question, otherwise than by pointing out the laws of nature which take place in vision; for by these the phænomena of vision must be regulated.

Therefore, I answer, First, That, by a law of nature, the rays of light proceed from every point of the object to the pupil of the eye, in straight lines; Secondly, That, by the laws of nature, the rays coming from any one point of the object to the various parts of the pupil, are so refracted as to meet again in one point of the retina; and the rays from different points of the object, first crossing each other,* and then proceeding to as many different points of the retina, form an inverted picture of the object.

So far the principles of optics carry us; and experience further assures us, that, if there is no such picture upon the retina, there is no vision; and that such as the picture on the retina is, such is the appear

It is marvellous how widely both natural philo. sophers and physiologists are at variance with regard to the point of the eye at which the rays cross each other. Some place this point in the cornea-some in the region of the pupil-some in the centre of the crystalline and some in the vitreous humour. Recent experiments, instituted for the purpose of determining its locality, and still unknown in this country, place it behind the crystalline lens. This is found to be at once the crossing point, both of the rays of light and of the line of visible direction, and the turning point on which the eye rolls.-H.

ance of the object, in colour and figure, distinctness or indistinctness, brightness or faintness.

It is evident, therefore, that the pictures upon the retina are, by the laws of nature, a mean of vision; but in what way they accomplish their end, we are totally igno rant. Philosophers conceive, that the impression made on the retina by the rays of light, is communicated to the optic nerve, and by the optic nerve conveyed to some part of the brain, by them called the sensorium; and that the impression thus conveyed to the sensorium is immediately perceived by the mind, which is supposed to reside there. But we know nothing of the seat of the soul: and we are so far from perceiving immediately what is transacted in the brain, that of all parts of the human body we know least about it. It is indeed very probable, that the optic nerve is an instrument of vision no less necessary than the retina ; and that some impression is made upon it, by means of the pictures on the retina. But of what kind this impression is, we know nothing.

There is not the least probability that there is any picture or image of the object either in the optic nerve or brain. The pictures on the retina are formed by the rays of light; and, whether we suppose, with some, that their impulse upon the retina causes some vibration of the fibres of the optic nerve, or, with others, that it gives motion to some subtile fluid contained in the nerve, neither that vibration nor this motion can resemble the visible object which is presented to the mind. Nor is there any probability that the mind perceives the pictures upon the retina. These pictures are no more objects of our percep tion, than the brain is, or the optic nerve. No man ever saw the pictures in his own eye, nor indeed the pictures in the eye of another, until it was taken out of the head and duly prepared.

It is very strange, that philosophers, of all ages, should have agreed in this notion, that the images of external objects are conveyed by the organs of sense to the brain, and are there perceived by the mind. Nothing can be more unphilosophical. For, First, This notion hath no foundation in fact and observation. Of all the organs of sense, the eye only, as far as we can discover, forms any kind of image of its object; and the images formed by the eye are not in the brain, but only in the bottom of the eye; nor are they at all perceived or felt by the mind.† Secondly, It is as difficult

This statement in its unqualified universality is altogether erroneous.-H.

This would require a second eye behind the retina; which eye would also see the images bent,

to conceive how the mind perceives images in the brain, as how it perceives things more distant. If any man will shew how the mind may perceive images in the brain, I will undertake to shew how it may perceive the most distant objects; for, if we give eyes to the mind, to perceive what is transacted at home in its dark chamber, why may we not make these eyes a little longer-sighted? and then we shall have no occasion for that unphilosophical fiction of images in the brain. In a word, the manner and mechanism of the mind's perception is quite beyond our comprehension; and this way of explaining it, by images in the brain, seems to be founded upon very gross notions of the mind and its operations; as if the supposed images in the brain, by a kind of contact, formed similar impressions or images of objects upon the mind, of which impressions it is supposed to be conscious.

We have endeavoured to shew, throughout the course of this inquiry, that the impressions made upon the mind by means of the five senses, have not the least resemblance to the objects of sense; and, therefore, as we see no shadow of evidence that there are any such images in the brain, so we see no purpose, in philosophy, that the supposition of them can answer. Since the picture upon the retina, therefore, is neither itself seen by the mind, nor produces any impression upon the brain or sensorium, which is seen by the mind, nor makes any impression upon the mind that resembles the object, it may still be asked, How this picture upon the retina causes vision?

Before we answer this question, it is proper to observe, that, in the operations of the mind, as well as in those of bodies, we must often be satisfied with knowing that certain things are connected, and invariably follow one another, without being able to discover the chain that goes between them. It is to such connections that we give the name of laws of nature; and when we say that one thing produces another by a law of nature, this signifies no more, but that one thing, which we call in popular language the cause, is constantly and invariably followed by another, which we call the effect; and that we know not how they are connected. Thus, we see it is a fact, that bodies gravitate towards bodies; and that this gravitation is regulated by certain mathematical proportions, according to the distances of the bodies from each other, and their quantities of matter. Being unable to discover the cause of this gravitation, and presuming that it is the immediate operation, either of the Author of nature,

as they are pictured on the concavity of that membrane.-H.

or of some subordinate cause, which we have not hitherto been able to reach, we call it a law of nature. If any philosopher should hereafter be so happy as to discover the cause of gravitation, this can only be done by discovering some more general law of nature, of which the gravitation of bodies is a necessary consequence. In every chain of natural causes, the highest link is a primary law of nature, and the highest link which we can trace, by just induction, is either this primary law of nature, or a necessary consequence of it. To trace out the laws of nature, by induction from the phænomena of nature, is all that true philosophy aims at, and all that it can ever reach.

There are laws of nature by which the operations of the mind are regulated, there are also laws of nature that govern the material system; and, as the latter are the ultimate conclusions which the human faculties can reach in the philosophy of bodies, so the former are the ultimate conclusions we can reach in the philosophy of minds.

To return, therefore, to the question above proposed, we may see, from what hath been just now observed, that it amounts to this-By what law of nature is a picture upon the retina the mean or occasion of my seeing an external object of the same figure and colour in a contrary position, and in a certain direction from the eye?

It will, without doubt, be allowed that I see the whole object in the same manner and by the same law by which I see any one point of it. Now, I know it to be a fact, that, in direct vision, I see every point of the object in the direction of the right line that passeth from the centre of the eye to that point of the object. And I know, likewise, from optics, that the ray of light that comes to the centre of my eye, passes on to the retina in the same direction. Hence, it appears to be a fact, that every point of the object is seen in the direction of a right line passing from the picture of that point on the retina, through the centre of the eye. As this is a fact that holds universally and invariably, it must either be a law of nature, or the necessary consequence of some more general law of nature; and, according to the just rules of philosophising, we may hold it for a law of nature, until some more general law be discovered, whereof it is a necessary consequence-which, I suspect, can never be done."

* A confirmation of this doctrine is drawn from the cases of Cheselden and others, in which no mental inversion of the objects is noticed, and which had it occurred, is too remarkable a phænomenon to have been overlooked. It is, indeed, generally asserted that