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 phenomenon 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 phenomenon be an argument for Dr. Porterfield's second principle, it must destroy the first.

The true cause of this phenomenon will be given afterward; 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, floorcloth, 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 SUBJECT.

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 phenomena 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 benefit 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 of the contrary opinions should have given instances in support of them, that are clear and indisputable; but I have not found one such instance on either side of the question in all the authors I have met with. I have given three instances from my own observation, in confirmation of Dr. Jurin's opinion, which admit of no doubt; and one which leans

rather to the other opinion, but is dubious. And here I must leave the matter to further observation.

In the 17th section, I have endeavoured to show, that the correspondence and sympathy of certain points of the two retina, into which we have resolved all the phenomena of single and double vision, is not, as Dr. Smith conceived, the effect of custom, nor changed by custom, but is a natural and original property of human eyes: and in the last section, that it is not owing to an original and natural perception of the true distance of objects from the eye, as Dr. Porterfield imagined. After this recapitulation, which is intended to relieve the attention of the reader, shall we enter into more theories upon this subject.

That of Dr. Briggs, first published in English, in the Philosophical Transactions, afterward 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 tention, 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 retina, which do not correspond, having different tentions 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 hydraulic engine, consisting of a bundle of pipes, which carry 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 ether 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 sensorium. And for all that we know of 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 carrying images, that a man would be tempted to invent a new one.

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? easy for an ingenious philosopher to fit the caliber of these empty tubes to the diameter 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

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 show 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 either? 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 sensorium 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 be ingenious, hoping that in time this hypothesis may be brought up into a system as philosophical, as that of animal spirits, or the vibration of the 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 earth, 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 hypothesis 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 seeing 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 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 thence 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 cameleon, do not meet, if I am rightly informed."

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.