concerning what appears to us, or does not appear? The question is, At what distance do the objects now in my eye appear? Do they all appear at one distance, as if placed in the concave surface of a sphere, the eye being in the centre? Every man, surely, may know this with certainty; and, if he will but give attention to the testimony of his eyes, needs not ask a philosopher how visible objects appear to him. Now, it is very true, that, if I look up to a star in the heavens, the other stars that appear at the same time, do appear in this manner: yet this phænomenon does not favour Dr Porterfield's hypothesis; for the stars and heavenly bodies do not appear at their true distances when we look directly to them, any more than when they are seen obliquely: and if this phænomenon be an argument for Dr Porterfield's second principle, it must destroy the first.

The true cause of this phænomenon will be given afterwards; therefore, setting it aside for the present, let us put another case. I sit in my room, and direct my eyes to the door, which appears to be about sixteen feet distant: at the same time, I see many other objects faintly and obliquely-the floor, floor-cloth, the table which I write upon, papers, standish, candle, &c. Now, do all these objects appear at the same distance of sixteen feet? Upon the closest attention, I find they do

not.

Section XIX.

OF DR BRIGGS'S THEORY, AND SIR ISAAC NEWTON'S CONJECTURE ON THIS SUB

JECT.

I am afraid the reader, as well as the writer, is already tired of the subject of single and double vision. The multitude of theories advanced by authors of great name, and the multitude of facts, observed without sufficient skill in optics, or related without attention to the most material and decisive circumstances, have equally contributed to perplex it.

In order to bring it to some issue, I have, in the 13th section, given a more full and regular deduction than had been given heretofore, of the phænomena of single and double vision, in those whose sight is perfect; and have traced them up to one general principle, which appears to be a law of vision in human eyes that are perfect and in their natural state.

In the 14th section, I have made it appear, that this law of vision, although excellently adapted to the fabric of human eyes, cannot answer the purposes of vision in some other animals; and therefore, very

probably, is not common to all animals. The purpose of the 15th and 16th sections. is, to inquire, Whether there be any deviation from this law of vision in those who squint ?-a question which is of real importance in the medical art, as well as in the philosophy of vision; but which, after all that hath been observed and. written on the subject, seems not to be ripe for a determination, for want of proper observations. Those who have had skill to make proper observations, have wanted opportunities; and those who have had opportunities, have wanted skill or attention. I have therefore thought it worth while to give a distinct account of the observations necessary for the determination of this question, and what conclusions may be drawn from the facts observed. I have likewise collected, and set in one view, the most conclusive facts that have occurred in authors, or have fallen under my own observation.

It must be confessed that these facts, when applied to the question in hand, make a very poor figure; and the gentlemen of the medical faculty are called upon, for the honour of their profession, and for the bene fit of mankind, to add to them.

All the medical, and all the optical writers upon the strabismus that I have met with, except Dr Jurin, either affirm, or take it for granted, that squinting persons see the object with both eyes, and yet see it single. Dr Jurin affirms that squinting persons never see the object with both eyes; and that, if they did, they would see it double. If the common opinion be true, the cure of a squint would be as pernicious to the sight of the patient, as the causing of a permanent squint would be to one who naturally had no squint; and, therefore, no physician ought to attempt such a cure, no patient ought to submit to it. But, if Dr Jurin's opinion be true, most young people that squint may cure themselves, by taking some pains; and may not only remove the deformity, but, at the same time, improve their sight. If the common opinion be true, the centres, and other points of the two retina, in squinting persons, do not correspond, as in other men, and Nature, in them, deviates from her common rule. But, if Dr Jurin's opinion be true, there is reason to think that the same general law of vision which we have found in perfect human eyes, extends also to those which squint.

It is impossible to determine, by reasoning, which of these opinions is true; or whether one may not be found true in some patients, and the other in others. Here, experience and observation are our only guides; and a deduction of instances is the only rational argument. It might, therefore, have been expected, that the patrons

hydraulic engine, consisting of a bundle of pipes, which carried to and fro a liquor called animal spirits. About the time of Dr Briggs, it was thought rather to be a stringed instrument, composed of vibrating chords, each of which had its proper tension and tone. But some, with as great probability, conceived it to be a wind instrument, which played its part by the vibrations of an elastic we her in the nervous fibrils.

These, I think, are all the engines into which the nervous system hath been moulded by philosophers, for conveying the images of sensible things from the organ to the

the matter, every man may freely choose which he thinks fittest for the purpose; for, from fact and experiment, no one of them can claim preference to another. Indeed, they all seem so unhandy engines for carryimages, that a man would be tempted to invent a new one.

..

That of Dr Briggs-first published in English, in the "Philosophical Transactions," afterwards in Latin, under the title of Nova Visionis Theoria," with a prefatory epistle of Sir Isaac Newton to the author-amounts to this, That the fibres of the optic nerves, passing from corresponding points of the retina to the thalami nervorum opticorum, having the same length, the same tension, and a similar situation, will have the same tone; and, therefore, their vibrations, excited by the impression of the rays of light, will be like unisons in music, and will present one and the same image to the mind: but the fibres passing from parts of the r tine which do not correspond, having different tensions and tones, will have discordant vibrations; and, therefore, present different images to the mind.

I shall not enter upon a particular examination of this theory. It is enough to observe, in general, that it is a system of conjectures concerning things of which we are entirely ignorant; and that all such theories in philosophy deserve rather to be laughed at, than to be seriously refuted.

From the first dawn of philosophy to this day, it hath been believed that the optic nerves are intended to carry the images of visible objects from the bottom of the eye to the mind; and that the nerves belonging to the organs of the other senses have a like office. But how do we know this? We conjecture it; and, taking this conjecture for a truth, we consider how the nerves may best answer this purpose. The system of the nerves, for many ages, was taken to be a

• This statement is far too unqualified.-H.

Since, therefore, a blind man may guess as well in the dark as one that sees, I beg leave to offer another conjecture touching the nervous system, which, I hope, will answer the purpose as well as those we have mentioned, and which recommends itself by its simplicity. Why may not the optic nerves, for instance, be made up of empty tubes, opening their mouths wide enough to receive the rays of light which form the image upon the retina, and gently conveying them safe, and in their proper order, to the very seat of the soul, until they flash in her face? It is easy for an ingenious phi losopher to fit the caliber of these empty tubes to the diameter of the particles of light, so as they shall receive no grosser kind of matter; and, if these rays should be in danger of mistaking their way, an expedient may also be found to prevent this; for it requires no more than to bestow upon the tubes of the nervous system a peristaltic motion, like that of the alimentary tube.

It is a peculiar advantage of this hypothesis, that, although all philosophers believe that the species or images of things are conveyed by the nerves to the soul, yet none of their hypotheses shew how this may be done. For how can the images of sound, taste, smell, colour, figure, and all sensible qualities, be made out of the vibrations of musical chords, or the undulations of animal spirits, or of ather? We ought not to suppose means inadequate to the end. Is it not as philosophical, and more intelligible, to conceive, that, as the stomach receives its food, so the soul receives her images by a kind of nervous deglutition? I might add, that we need only continue this peristaltic motion of the nervous tubes from the sens rium to the extremities of the nerves that serve the muscles, in order to account for muscular motion.

Thus Nature will be consonant to herself; and, as sensation will be the conveyance of the ideal aliment to the mind, so muscular motion will be the expulsion of the recrementitious part of it. For who can deny, that the images of things conveyed by sensation, may, after due concoction, become fit to be thrown off by muscular motion? I only give hints of these things to the ingenious, hoping that in time this hypothesis may be wrought up into a system as truly philosophical as that of animal spirits, or the vibration of nervous fibres. To be serious: In the operations of nature, I hold the theories of a philosopher, which are unsupported by fact, in the same estimation with the dreams of a man asleep, or the ravings of a madman. We laugh at the Indian philosopher, who, to account for the support of the carth, contrived the hypothesis of a huge elephant, and, to support the elephant, a huge tortoise. If we will candidly confess the truth, we know as little of the operation of the nerves, as he did of the manner in which the earth is supported; and our hypotheses about animal spirits, or about the tension and vibrations of the nerves, are as like to be true, as his about the support of the earth. His elephant was a hypothesis, and our hypotheses are elephants. Every theory in philosophy, which is built on pure conjecture, is an elephant; and every theory that is supported partly by fact, and partly by conjecture, is like Nebuchadnezzar's image, whose feet were partly of iron and partly of clay.

The great Newton first gave an example to philosophers, which always ought to be, but rarely hath been followed, by distinguishing his conjectures from his conclusions, and putting the former by themselves, in the modest form of queries. This is fair and legal; but all other philosophical traffic in conjecture ought to be held contraband and illicit. Indeed, his conjectures have commonly more foundation in fact, and more verisimilitude, than the dogmatical theories of most other philosophers; and, therefore, we ought not to omit that which he hath offered concerning the cause of our secing objects single with two eyes, in the 15th query annexed to his "Optics.' "Are not the species of objects seen with both eyes, united where the optic nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and after union going thence into the brain in the nerve which is on the right side of the head, and the fibres on the left side of both nerves uniting in the same place, and after union going into the brain in the nerve which is on the left side of the head, and these two nerves meeting in the brain in such a manner that their fibres

make but one entire species or picture, half of which on the right side of the sensorium comes from the right side of both eyes through the right side of both optic nerves, to the place where the nerves meet, and from theuce on the right side of the head into the brain, and the other half on the left side of the sensorium comes, in like manner, from the left side of both eyes? For the optic nerves of such animals as look the same way with both eyes (as men,. dogs, sheep, oxen, &c.) meet before they come into the brain; but the optic nerves of such animals as do not look the same way with both eyes (as of fishes, and of the chameleon) do not meet, if I am rightly in- formed."

I beg leave to distinguish this query into two, which are of very different natures; one being purely anatomical, the other relating to the carrying species or pictures of visible objects to the sensorium.

The first question is, Whether the fibres coming from corresponding points of the two retina do not unite at the place where the optic nerves meet, and continue united from thence to the brain; so that the right optic nerve, after the meeting of the two nerves, is composed of the fibres coming from the right side of both retina, and the left, of the fibres coming from the left side of both retina?

This is undoubtedly a curious and rational question; because, if we could find ground from anatomy to answer it in the affirmative, it would lead us a step forward in discovering the cause of the correspondence and sympathy which there is between certain points of the two retina. For, although we know not what is the particular function of the optic nerves, yet it is probable that some impression made upon them, and communicated along their fibres, is necessary to vision; and, whatever be the nature of this impression, if two fibres are united into one, an impression made upon one of them, or upon both, may probably produce the same effect. Anatomists think it a sufficient account of a sympathy between two parts of the body, when they are served by branches of the same nerve; we should, therefore, look upon it as an important discovery in anatomy, if it were found that the same nerve sent branches to the corresponding points of the retina.

But hath any such discovery been made? No, not so much as in one subject, as far as I can learn ; but, in several subjects, the contrary seems to have been discovered. Dr Porterfield hath given us two cases at length from Vesalius, and one from Cæsalpinus, wherein the optic nerves, after touching one another as usual, appeared to be reflected back to the same side whence they came, without any mixture of their

fibres. Each of these persons had lost an eye some time before his death, and the optic nerve belonging to that eye was shrunk, so that it could be distinguished from the other at the place where they met. Another case, which the same author gives from Vesalius, is still more remarkable; for in it the optic nerves did not touch at all; and yet, upon inquiry, those who were most familiar with the person in his lifetime, declared that he never complained of any defect of sight, or of his seeing objects double. Diemerbroeck tells us, that Aquapendens [ab Aquapendente] and Valverda likewise affirm, that they have met with subjects wherein the optic nerves did not touch.

As these observations were made before Sir Isaac Newton put this query, it is uncertain whether he was ignorant of them, or whether he suspected some inaccuracy in them, and desired that the matter might be more carefully examined. But, from the following passage of the most accurate Winslow, it does not appear that later observations have been more favourable to his conjecture. "The union of these (optic) nerves, by the small curvatures of their cornua, is very difficult to be unfolded in human bodies. This union is commonly found to be very close; but, in some subjects, it seems to be no more than a strong adhesion-in others, to be partly made by an intersection or crossing of fibres. They have been found quite separate; and, in other subjects, one of them has been found to be very much altered both in size and colour through its whole passage, the other remaining in its natural state.'

"

When we consider this conjecture of Sir Isaac Newton by itself, it appears more ingenious, and to have more verisimilitude, than anything that has been offered upon the subject; and we admire the caution and modesty of the author, in proposing it only as a subject of inquiry: but when we compare it with the observations of @natomists which contradict it,† we are naturally

See Meckel's" Pathologische Anatomie," 1., p. 399.-H. + Anatomists are now nearly agreed, that, in the normal state, there is a partial decussation of the human optic nerve. Soemmering, Treviranus, Rudolphi, Johannes Mueller, Langenbeck, Magendie, Mayo, &c., are paramount authority for the fact. donot know whether the observation has been made, that the degree of decussat on in different animals is exactly in the inverse ratio of what we might have been led, at first sight, theoretically to anticipate. In proportion as the convergence is complete—i. e., where the axis of the field of vision of the severareyes coincides with the axis of the field of vision common to both, as in men and apes-there we find the decussation most partial and obscure; whereas, in the lower animals, in proportion as we find the fields of the two eyes exclusive of each other, and where, consequently, the necessity of bringing the two organs into unison might seem abolished, there, however, we find the crossing of the optic fibres complete. In fishes, accordingly, it is distinct and isolated; in birds, it takes

led to this reflection, That, if we trust to the conjectures of men of the greatest genius in the operations of nature, we have only the chance of going wrong in an ingc

nious manner.

The second part of the query is, Whether the two species of objects from the two eyes are not, at the place where the optic nerves meet, united into one species or picture, half of which is carried thence to the sensorium in the right optic nerve, and the other half in the left? and whether these two halves are not so put together again at the sensorium, as to make one species or picture?

Here it seems natural to put the previous question, What reason have we to believe that pictures of objects are at all carried to the sensorium, either by the optic nerves, or by any other nerves? Is it not possible that this great philosopher, as well as many of a lower form, having been led into this opinion at first by education, may have continued in it, because he never thought of calling it in question? I confess this was my own case for a considerable part of my life. But since I was led by accident to think seriously what reason I had to believe it, I could find none at all. It seems to be a mere hypothesis, as much as the Indian philosopher's elephant. I am not conscious of any pictures of external objects in my sensorium, any more than in my stomach: the things which I perceive by my senses, appear to be external, and not in any part of the brain; and my sensations, properly so called, have no resemblance of external objects.

The conclusion from all that hath been said, in no less than seven sections, upon our seeing objects single with two eyes, is this-That, by an original property of human eyes, objects painted upon the centres of the two retine, or upon points similarly situate with regard to the centres, appear in the same visible place; that the most plausible attempts to account for this property of the eyes, have been unsuccessful; and, therefore, that it must be either a primary law of our constitution, or the consequence of some more general law, which is not yet discovered.

We have now finished what we intended to say, both of the visible appearances of things to the eye, and of the laws of our constitution by which those appearances

more the appearance of an interlacement; in the mammalia, that of a fusion of substance. A second consideration, however, reconciles theory and observ. a'ion. Some, however, as Woolaston, make the parallel motion of the eyes to be dependent on the connection of the optic nerves; and, besides experiments, there are various pathological ca-es in favour of Magendie's opinion, that the fifth pair are the nerves on, which the energies of sight, hearing, smell, and taste are proximately and principally dependent.-H.

are exhibited. But it was observed, in the beginning of this chapter, that the visible appearances of objects serve only as signs of their distance, magnitude, figure, and other tangible qualities. The visible appearance is that which is presented to the mind by nature, according to those laws of our constitution which have been explained. But the thing signified by that appearance, is that which is presented to the mind by

custom.

When one speaks to us in a language that is familiar, we hear certain sounds, and this is all the effect that his discourse has upon us by nature; but by custom we understand the meaning of these sounds; and, therefore, we fix our attention, not upon the sounds, but upon the things signified by them. In like manner, we see only the visible appearance of objects by nature; but we learn by custom to interpret these appearances, and to understand their meaning. And when this visual language is learned, and becomes familiar, we attend only to the things signified; and cannot, without great difficulty, attend to the signs by which they are presented. The mind passes from one to the other so rapidly and so familiarly, that no trace of the sign is left in the memory, and we seem immediately, and without the intervention of any sign, to perceive the thing signified.

When I look at the apple-tree which stands before my window, I perceive, at the first glance, its distance and magnitude, the roughness of its trunk, the disposition of its branches, the figure of its leaves and fruit.

I seem to perceive all these things immediately. The visible appearance which presented them all to the mind, has entirely escaped me; I cannot, without great difficulty, and painful abstraction, attend to it, even when it stands before me. Yet it is certain that this visible appearance only is presented to my eye by nature, and that I learned by custom to collect all the rest from it. If I had never seen before now, I should not perceive either the distance or tangible figure of the tree; and it would have required the practice of seeing for many months, to change that original perception which nature gave me by my eyes, into that which I now have by custom.

The objects which we see naturally and originally, as hath been before observed, have length and breadth, but no thickness nor distance from the eye. Custom, by a kind of legerdemain, withdraws gradually these original and proper objects of sight, and substitutes in their place objects of touch, which have length, breadth, and thickness, and a determinate distance from the eye. By what means this change is brought about, and what principles of the

human mind concur in it, we are next to inquire.

Section XX.

OF PERCEPTION IN GENERAL.

Sensation, and the perception of external objects by the senses, though very different in their nature, have commonly been considered as one and the same thing. The purposes of common life do not make it necessary to distinguish them, and the received opinions of philosophers tend rather to confound them; but, without attending carefully to this distinction, it is impossible to have any just conception of the operations of our senses. The most simple operations of the mind, admit not of a logical definition: all we can do is to describe them, so as to lead those who are conscious of them in themselves, to attend to them, and reflect upon them; and it is often very difficult to describe them so as to answer this intention.

The same mode of expression is used to denote sensation and perception; and, therefore, we are apt to look upon them as things of the same nature. Thus, I feel a pain; I see a tree: the first denoteth a sensation, the last a perception. The grammatical analysis of both expressions is the same:

Nothing in the compass of inductive reasoning appears more satisfactory than Berkeley's demon. stration of the necessity and manner of our learning, by a slow process of observation and comparison alone, the connection between the perceptions of vision and touch, and, in general, all that relates to the distance and real m guitud of external things. But, although the same necessity seems in theory equally incumbent on the lower animals as on man, yet this theory is provokingly-and that by the most manifest exerience-found totally at fault with regard to them; for we find that all the animals who possess at birth the power of regulated motion (and these are those only through whom the truth of the theory can be brought to the test of a decisive experiment) possess also from birth the whole apprehension of distance, &c, which they are ever known to exhibit. The solution of this difference, by a resort to instinct, sunsatisfactory; for instinct is, in fact, an occult principle- a kind of natural rev 1. ation and the hypothesis of instinct, therefore only a confession of our ignorance; and, at the same time, if instinct be allowed in the lower animals, how can we determine whether and how far instinct may not in like manner operate to the same re-ult in man? I have discovered, and, by a wide induc. ion, esta! lished, that the power of regulated mo. tion at birth is, in all animals, governed by the de. velopement, at that period, of the cerebel um, in proportion to the brain proper. Is this law to be exte ded to the faculty of determining distances, &c., by sight? -H.

On the distinction of Sensation proper, from Powers," Essay 11, chap. 16, and Note D. Perception prope, see "Essays on the Intellectual Reid himself, especially in this work, has not been always rigid in observing their discrimination.-H.

Not only are they different, but-what has escaped our philos phers-the law of their mamte-tation is, that, while both are co-existent, each is always in the inverse ratio of the other. Perception is the objec tive, Sensation the subjective, element. This by the way.-H.