It may be taken for a general rule, That things which are produced by custom may be undone or changed by disuse or by contrary custom. On the other hand, it is a strong argument that an effect is not owing to custom, but to the constitution of nature, when a contrary custom, long continued, is found neither to change nor weaken it. I take this to be the best rule by which we can determine the question presently under consideration. I shall there- fore mention two facts brought by Dr. Smith, to prove that the corresponding points of the retince have been changed by custom; and then I shall mention some facts tending to prove that there are corresponding points of the retina of the eyes originally, and that custom produces no change in them.
" One fact is related upon the authority of Martin Folkes, Esq. who was informed by Dr. Hepburn of Lynn, that the Reverend Mr. Foster of Clinchwarton, in that neighbourhood, having been blind for some years of a gutta serena, was restored to sight by salivation: and that upon his first beginning to see, all objects appeared to him double; but afterwards the two appearances ap- proaching by degrees, he came at last to see single, and as distinctly as he did before he was blind."
Upon this case I observe, first, That it does not prove any change of the corresponding points of the eyes, unless we suppose what is not affirmed, that Mr. Foster directed SECT. XVII.] OF SEEING. 183 his eyes to the object at first, when he saw double, with the same accuracy and in the same manner that he did afterwards, when he saw single. Secondly, If we should suppose this, no account can be given, why at first the two appearances should be seen at one certain angular distance rather than another; or why this angular distance should gradually decrease, until at last the appearances coincided. How could this effect be produced by cus- tom? But, thirdly, Every circumstance of this case may be accounted for, on the supposition that Mr. Foster had corresponding points on the retinae of his eyes from the time he began to see, and that custom made no change with regard to them. We need only further suppose, what is common in such cases, that by some years blind- ness he had lost the habit of directing his eyes accurately to an object, and that he gradually recovered this habit when he came to see.
The second fact mentioned by Dr. Smith is taken from Mr. Cheselden's Anatomy; and is this: " A gentleman who, from a blow on the head, had one eye distorted, found every object appear double; but by degrees the most familiar ones became single; and in time all objects became so, without any amendment of the distortion."
I observe here, that it is not said that the two appear- ances gradually approached, and at last united, without any amendment of the distortion. This would indeed have been a decisive proof of a change in the corresponding points of the retinae; and yet of such a change as could not be accounted for from custom. But this is not said; and if it had been observed, a circumstance so remarkable would have been mentioned by Mr. Cheselden, as i' in the other case by Dr. Hepburn. We may therefore take it for granted that one of the appearances vanished by degrees, without approaching to the other. And this I conceive might happen several ways. First, The sight of the distorted eye might gradually decay by the hurt; so the appearances presented by that eye would gradually vanish. Secondly, A small and unperceived change in the manner of directing the eyes might occasion his not 184 OF THE HUMAN MIND. [CHAP. VI.
seeing the object with the distorted eye, as appears from Sect. 15. Art. 10. Thirdly, By acquiring the habit of directing one and the same eye always to the object, the faint and oblique appearance, presented by the other eye, might be so little attended to when it became familiar, as not to be perceived. One of these causes, or more of them concurring, might produce the effect mentioned, with- out any change of the corresponding points of the eyes.
For these reasons, the facts mentioned by Dr. Smith, although curious, seem not to be decisive.
The following facts ought to be put in the opposite scale. First, In the famous case of the young gentleman couched by Mr. Cheselden, after having had cataracts on both eyes until he was thirteen years of age, it appears, that he saw objects single from the time he began to see with both eyes. Mr. Cheselden's words are: " And now being lately couched of his other eye, he says, that objects at first appeared large to this eye, but not so large as they did at first to the other; and looking upon the same object with both eyes, he thought it looked about twice as large as with the first couched eye only, but not double that we can anywise discover."
Secondly, The three young gentlemen mentioned in the last section, who had squinted, as far as I know, from infancy, as they learned to direct both eyes to an object, saw it single. In these four cases, it appears evident that the centres of the retince corresponded originally, and before custom could produce any such effect: for Mr. Che- selden's young gentleman had never been accustomed to see at all before he was couched; and the other three had never been accustomed to direct the axes of both eyes to the object.
Thirdly, From the facts recited in Sect. 13, it appears, That from the time we are capable of observing the pheno- mena of single and double vision, custom makes no change in them.
I have amused myself with such observations for more than thirty years; and in every case wherein I saw the object double at first, I see it so to this day, notwithstand- SECT. XVII.] OF SEEING. 185 ing the constant experience of its being single. In other cases where I know there are two objects, there appears only one, after thousands of experiments.
Let a man look at a familiar object through a polyhe- dron or multiplyingglass every hour of his life, the number of visible appearances will be the same at last as at first: nor does any number of experiments, or length of time, make the least change.
Effects produced by habit must vary according as the acts by which the habit is acquired are more or less fre- quent: but the phenomena of single and double vision are so invariable and uniform in all men, are so exactly regu- lated by mathematical rules, that I think we have good reason to conclude, that they are not the effect of custom, but of fixed and immutable laws of nature.
SECT. XVIII.
Of Dr. PorterfieltTs Account of Single and Double Vision.
BISHOP BERKELEY and Dr. Smith seem to attribute too much to custom in vision; Dr. Porterfield too little.
This ingenious writer thinks, that by an original law of our nature, antecedent to custom and experience, we per- ceive visible objects in their true place, not only as to their direction, but likewise as to their distance from the eye: and therefore he accounts for our seeing objects single with two eyes in this manner. Having the faculty of perceiving the object with each eye in its true place, we must perceive it with both eyes in the same place; and consequently must perceive it single.
He is aware that this principle, although it accounts for our seeing objects single with two eyes, yet does not at all account for our seeing objects double: and whereas other writers oil this subject take it to be a sufficient cause for double vision that we have two eyes, and only find it difficult to assign a cause for single vision; on the con- 186 OF THE HUMAN MIND. [CHAP. VI.
trary, Dr. Porterfield's principle throws all the difficulty on the other side.
Therefore, in order to account for the phenomena of double vision, he advances another principle, without sig- nifying whether he conceives it to be an original law of our nature, or the effect of custom. It is, That our natu- ral perception of the distance of objects from the eye is not extended to all the objects that fall within the field of vision, but limited to that which we directly look at; and that the circumjacent objects, whatever be their real dis- tance, are seen at the same distance with the object we look at; as if they were all in the surface of a sphere whereof the eye is the centre.
Thus, single vision is accounted for by our seeing the true distance of an object which we look at; and double vision, by a false appearance of distance in objects which we do not directly look at.
We agree with this learned and ingenious author, that it is by a natural and original principle that we see visible objects in a certain direction from the eye, and honour him as the author of this discovery: but we cannot assent to either of those principles by which he explains single and double vision, for the following reasons: 1. Our having a natural and original perception of the distance of objects from the eye appears contrary to a well attested fact: for the young gentleman couched by Mr. Cheselden imagined, at first, that whatever he saw touched his eye, as what he felt touched his hand.
2. The perception we have of the distance of objects from the eye, whether it be from nature or custom, is not so accurate and determinate as is necessary to produce single vision. A mistake of the twentieth or thirtieth part of the distance of a small object, such as a pin, ought, according to Dr. Porterfield's hypothesis, to make it ap- pear double. Very few can judge of the distance of a visible object with such accuracy. Yet we never find double vision produced by mistaking the distance of the object. There are many cases in vision, even with the naked eye, wherein we mistake the distance of an object SECT. XVIII.] OF SEEING. 187 by one half or more: why do we see such objects single? When I move my spectacles from my eyes towards a small object, two or three feet distant, the object seems to approach so as to be seen at last at about half its real distance; but it is seen single at that apparent distance, as well as when we see it with the naked eye at its real dis- tance. And when we look at an object with a binocular telescope, properly fitted to the eyes, we see it single, while it appears fifteen or twenty times nearer than it is. There are then few cases wherein the distance of an object from the eye is seen so accurately as is necessary for single vision, upon this hypothesis: This seems to be a conclu- sive argument against the account given of single vision. We find, likewise, that false judgments or fallacious ap- pearances of the distance of an object do not produce double vision. This seems to be a conclusive argument against the account given of double vision.
3. The perception we have of the linear distance ef objects, seems to be wholly the effect of experience. This I think hath been proved by Bishop Berkeley and by Dr. Smith; and when we come to point out the means of judging of distance by sight, it will appear that they are all furnished by experience.
4. Supposing that, by a law of our nature, the distance of objects from the eye were perceived most accurately, as well as their direction, it will not follow that we must see the object single. Let us consider what means such a law of nature would furnish for resolving the question, Whether the objects of the two eyes are in one and the same place, and consequently are not two, but one?
Suppose then two right lines, one drawn from the centre of one eye to its object, the other drawn in like manner from the centre of the other eye to its object: The law of nature gives us the direction or position of each of these right lines, and the length of each; and this is all that it gives. These are geometrical data, and we may learn from geometry what is determined by their means. It is then determined by these data, Whether the two risht lines terminate in one and the same point, or not? No truly. In order to determine this, we must have three 188 OF THE HUMAN MIND. [CHAP. VI.
other data. \Ve must know whether the two right lines are in one plane: we must know what angle they make, and we must know the distance between the centres of the eyes. And, when these things are known, we must apply the rules of trigonometry, before we can resolve the ques- tion, Whether the objects of the two eyes are in one and the same place; and consequently whether they are two or one?
5. That false appearance of distance into which double vision is resolved cannot be the effect of custom, for con- stant experience contradicts it: neither hath it the fea- tures of a law of nature; because it does not answer any good purpose, nor indeed any purpose at all but to de- ceive us. But why should we seek for arguments, in a question concerning what appears to us, or does not ap- pear? The question is, At what distance do the objects now in my eye appear? Do they all appear at one dis- tance, 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 ap- pear 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 after- wards; 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 dis- tant: 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.
OF SEEING. 189 SECT. XIX.
Of Dr. Briggs's Theory, and Sir Isaac Newton's Con- jecture 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 com- mon 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 oppor- tunities 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 ques- tion, 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.
190 OF THE HUMAN MIND. [CHAP. VI.
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 re- move the deformity, but at the same time improve their sight. If the common opinion be true, the centres and other parts 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 laws of vision which we have found hi 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 expe- rience and observation are our only guides; and a deduc- tion of instances is the only rational argument. It might therefore have been expected, that the patrons of the con- trary 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 authours 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.
SECT. XIX.] OF SEEING. 191 In the 17th Section, I have endeavoured to show that the correspondence and sympathy of certain points of the two retinae, into which we have resolved all the pheno- mena of single and double vision, is not, as Dr. Smith con- ceived, the effect of custom, nor can be changed by cus- tom, 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 recapi- tulation, 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, 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 retinae wliich do not correspond, having different tensions and tones, will have discordant vibrations; and therefore pre- sent 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 re- futed.
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 192 OF THE HUMAN MIND. [CHAP. VI.
taken to be an hydraulic engine, consisting of a bundle of pipes, which carry to and fro a liquor called animal spirits About the tune 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 great probability, conceived it to be a wind instrument, which played its part by the vibrations of an elastic t: -t -; in the nervous fibrils.
These, I think, are all the engines into which the ner- vous system hath been moulded by philosophers, for con- veying the images of sensible things from the organ to th- sensorium. And for all that we know of the ma every man may freely choose which he thinks fittest tm the purpose; for, from fact and experiment, no on, them can claim preference to another. Indeed, th< 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 thi 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 philosopher to fit the calibre of tht M empty tubes to the diameter of the particles of light, so as they shall receive no grosser kind of matter. And if t rays should be in danger of mistaking their way, an expe- dient may also be found to prevent this. For it requires no more than to bestow upon the tubes of the nervous sys- tem, 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 show how this may be done. For how can images of sound, taste, smell, colour, figure, and all SECT. XIX.] OF SEEING. 193 sensible qualities, be made out of the vibrations of musical chords, or the undulations of animal spirits, or of ether? We ought not to suppose means inadequate to the end. Is it not as philosophical, and more intelligible, to con- ceive, 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 sen- sation will be the conveyance of the ideal aliment to the mind, so muscular motion will be the expulsion of the re- crementitious 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 mo- tion? 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 earth, contrived the hypothesis of a huge elephant, and to sup- port the elephant a huge tortoise. If we will candidly confess the truth, we know as little of the operations of the nerves as he did of the manner in which the earth is sup- ported; 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 an hypothesis, and our hypotheses are elephants. Every theory in philosophy, which is built on pure con- jecture, is an elephant; and every theory that is supported partly by fact and partly by conjecture is like Nebuchad- nezzar's image, whose feet were partly of iron and partly of clay.
The great Newton first gave an example to philoso- K 194 OP THE HUMAN MIND. [CHAP. VI.
phers, which always ought to be, but rarely hath been fol- lowed, by distinguishing his conjectures from his conclu- sions, and putting the former by themselves, in the modest form of queries. This is fair and legal; but all other phi- losophical traffic in conjecture ought to be held contraband and illicit. Indeed his conjectures have commonly more foundation in fact, and more verisimilitude, than the dog- matical theories of most other philosophers; and there- fore we ought not to omit that which he hath offered con- cerning the cause of our seeiqg 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 thence to 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 cha- meleon) 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.
The first question is, Whether the fibres coming from corresponding points of the two retinae do not unite at the place where the optic nerves meet, and continue united SECT. XIX.] OF SEEING. 195 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 retince?
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 dis- covering the cause of the correspondence and sympathy which there is between certain points of the two retince. 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 pro- duce 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 retinae.