When we look at an object, the circumjacent objects may be seen at the same time, although more obscurely and indistinctly: for the eye hath a considerable field of vision, which it takes in at once. But we attend only to he object we look at. The other objects which fall within the field of vision are not attended to; and therefore are as if they were not seen. If any of them draws our atten- tion, it naturally draws the eyes at the same time; for, in the common course of life, the eyes always follow the at- tention: or if, at any time, in a reverie, they are separated from it, we hardly at that time see what is directly before us. Hence we may see the reason, why the man we are speaking of thinks that he never before saw an object louble. When he looks at any object he sees it single, and takes no notice of other visible objects at that time, whe- ther they appear single or double. If any of them draws his attention, it draws his eyes at the same time; and as soon as the eyes are turned towards it it appears single. But in order to see things double, at least in order to have any reflection or remembrance that he did so, it is necessary that he should look at one object, and at the same time attend to the faint appearance of other objects which are within the field of vision. This is a practice which per- haps he never used nor attempted; and, therefore, he does not recollect that ever he saw an object double. But when lie is put upon giving this attention, he immediately sees ob- jects double in the same manner, and with the very same circumstances as they who have been accustomed, for the greatest part of their lives, to give this attention.
There are many phenomena of a similar nature which show that the mind may not attend to, and thereby, in some sort, not perceive objects that strike the senses. I 158 OF THE HUMAN MIND. [CHAP. VI.
had occasion to mention several instances of this hi the second chapter; and I have been assured, by persons of the best skill in music, that in hearing a tune upon the harpsichord, when they give attention to the treble, they do not hear the bass; and when they attend to the bass, they do not perceive the air of the treble. Some persons are so near sighted that, in reading, they hold the book to one eye, while the other is directed to other objects. Such persons acquire the habit of attending in this case to the objects of one eye, while they give no attention to those of the other.
6. It is observable, that in all cases wherein we see an object double, the two appearances have a certain position with regard to one another, and a certain apparent or an- gular distance. This apparent distance is greater or less in different circumstances; but in the same circumstances it is always the same, not only to the same, but to different persons.
Thus, in the experiment above mentioned, if twenty dif- ferent persons, who see perfectly with both eyes, shall place their ringer and the candle at the distances above expressed, and hold their heads upright; looking; at the finger, they will see two candles, one on the right, another on the left. That which is seen on the right, is seen by the right eye, and that which is seen on the left, by the left eye; and they will see them at the same apparent dis- tance from each other. If again they look at the candle, they will see two fingers, one on the right and the other on the left; and all will see them at the same apparent dis- tance; the finger towards the left being seen with the right eye, and the other by the left. If the head is laid horizon- tally to one side, other circumstances remaining the same, one appearance of the object seen double will be directly above the other. In a word, vary the circumstances as you please, and the appearances are varied to all the spec- tators in one and the same manner.
7. Having made many experiments in order to ascer- tain the apparent distance of the two appearances of an object seen double, I have found that in all cases this ap- SECT. XIII.] OF SEEING. 159 parent distance is proportioned to the distance between the point of the retina, where the picture is made in one eye, and the point which is situated similarly to that on which the picture is made on the other eye. So that as the apparent distance of two objects seen with one eye is pro- portioned to the arch of the retina which lies between their pictures: in like manner, when an object is seen dou- ble with the two eyes, the apparent distance of the two appearances is proportioned to the arch of either retina which lies between the picture in that retina, and the point corresponding to that of the picture in the other retina, 8. As in certain circumstances we invariably see one object appear double, so in others we as invariably see two objects unite into one; and, in appearance, lose their duplicity. This is evident in the appearance of the bino- cular telescope. And the same thing happens when any two similar tubes are applied to the two eyes in a parallel direction; for in this case we see only one tube. And if two shillings are placed at the extremities of the two tubes, one exactly in the axis of one eye, and the other at the axis of the other eye, we shall see but one shilling. Tf two pieces of coin, or other bodies, of different colour, and of different figure, be properly placed in the two axes of the eyes, and at the extremities of the tubes, we shall see both the bodies in one and the same place, each as it were spread over the other, without hiding it; and the colour will be that which is compounded of the two colours.
9. From these phenomena, and from all the trials I have been able to make, it appears evident, that in perfect hu- man eyes, the centres of the two retinae correspond and harmonize with one another; and that every other point in one retina doth correspond and harmonize with the point which is similarly situate in the other; in such manner that pictures falling on the corresponding points of the two retince show only one object, even when there are really two: and pictures falling upon points of the retina: which do not correspond show us two visible appearances, although there be but one object. So that pictures, upon corresponding points of the two retinae present the same 160 OF THE HUMAN MIND. [CHAP. VI.
appearance to the mind as if they had both fallen upon the same point of one retina; and pictures upon points of the two retinae, which do not correspond, present to the mind the same apparent distance and position of two objects, as if one of those pictures was carried to the point corres- ponding to it in the other retina. This relation and sym- pathy between corresponding points of the two retinae, I do not advance as an hypothesis, but as a general fact or phenomenon of vision. All the phenomena before men- tioned of single or double vision lead to it, and are neces- sary consequences of it. It holds true invariably in all perfect human eyes, as far as I am able to collect from innumerable trials of various kinds made upon my own eyes, and many made by others at my desire. Most of the hypotheses that have been contrived to resolve the phenomena of single and double vision suppose this gene- ral fact, while their authors were not aware of it. Sir Isaac Newton, who was too judicious a philosopher and too accurate an observer to have offered even a conjecture which did not tally with the facts that had fallen under his observation, proposes a query with respect to the cause of it, Optics, guer. 15. The judicious Dr. Smith, in his Optics, lib. 1, § 137, hath confirmed the truth of this ge- neral phenomenon from his own experience, not only as to the apparent unity of objects whose pictures fall upon the corresponding points of the retinae, but also as to the appa- rent distance of the two appearances of the same object when seen double.
This general phenomenon appears therefore to be founded upon a very full induction, which is all the evidence we can have for a fact of this nature. Before we make an end of this subject, it will be proper to inquire, first, Whe- ther those animals, whose eyes have an adverse position in their heads, and look contrary ways, have such corres- ponding points in their retinas? Secondly, What is the position of the corresponding points in imperfect human eyes, I mean in those that squint? And, in the last place, Whether this harmony of the corresponding points in the retinae be natural and original, or the effects of custom?
SECT. XIII.] OF SEEING. 161 And if it is original, Whether it can be accounted for by any of the laws of nature already discovered? or whether it is itself to be looked upon as a law of nature, and a part of the human constitution?
SECT. XIV.
Of the Laws of Vision in Brute Animals.
IT is the intention of nature, in giving eyes to animals, that they may perceive the situation of visible objects, or the direction in which they are placed: it is probable, therefore, that, in ordinaiy cases, every animal, whether it has many eyes or few, whether of one structure or of an- other, sees objects single, and in their true and proper direction. And since there is a prodigious variety in the structure, the motions, and the number of eyes hi different animals and instincts, it is probable that the laws by which vision is regulated are not the same in all, but various, adapted to the eyes which nature hath given them.
Mankind naturally turn their eyes always the same way, so that the axes of the two eyes meet in one point. They naturally attend to or look at that object only which is placed in the point where the axes meet. And whether the object be more or less distant, the configuration of the eye is adapted to the distance of the object, so as to form a distinct picture of it.
When we use our eyes in this natural way, the two pic- tures of the object we look at are formed upon the centres of the two retina; and the two pictures of any contiguous object are formed upon the points of the retina winch are similarly situate with regard to the centres. Therefore, in order to our seeing objects single, and in their proper direction, with two eyes, it is sufficient that we be so con- stituted that objects whose pictures are formed upon the centres of the two retina, or upon points similarly situated with regard to these centres, shall be seen hi the same vU 162 OF THE HUMAN MIND. [CHAP. VI.
sible place. And this is the constitution which nature hath actually given to human eyes.
When we distort our eyes from their parallel direction, which is an unnatural motion, but may be learned by prac- tice; or when we direct the axes of the two eyes to one point, and at the same time direct our attention to some visible object much nearer or much more distant than that point, which is also unnatural, yet may be learned; in these cases, and in these only, we see one object double, or two objects confounded in one. In these cases, the two pictures of the same object are formed upon points of the retina, which are not similarly situate, and so the object is seen double; or the two pictures of different objects are formed upon points of the retinas, which are similarly situ- ate, and so the two objects are seen confounded in one place.
Thus it appears, that the laws of vision in the human constitution are wisely adapted to the natural use of human eyes, but not to that use of them which is unnatural. We see objects truly when we use our eyes in the natural way; but have false appearances presented to us when we use them in a way that is unnatural. We may reason- ably think, that the case is the same with other animals. But is it not unreasonable to think, that those animals which naturally turn one eye towards one object, and an- other eye towards another object, must thereby have such false appearances presented to them, as we have when we do so against nature?
Many animals have their eyes by nature placed adverse and immovable, the axes of the two eyes being always directed to opposite points. Do objects painted on the centres of the two retina appear to such animals as they do to human eyes, in one and the same visible place? I think it is highly probable that they do not; and that they appear, as they really are, in opposite places.
If we judge from analogy in this case, it will lead us to think that there is a certain correspondence between points of the two retina in such animals, but of a different kind SECT. XIV.] OF SEEING. 163 from that which we have found in human eyes.. The cen- tre of one retina will correspond with the centre of the other, in such manner, that the objects whose pictures are formed upon these corresponding points, shall appear not to be in the same place, as in human eyes, but in opposite places. And in the same manner will the superior part of one retina correspond with the inferior part of 4he other, and the anterior part of one with the posterior part of the other.
Some animals, by nature, turn their eyes with equal facility, either the same way or different ways, as we turn our hands and arms. Have such animals corresponding points in their retinte, and points which do not correspond, as the humankind has? I think it is probable that they have not; because such a constitution in them could serve no other purpose but to exhibit false appearances.
If we judge from analogy, it will lead us to think, that as such animals move their eyes in a manner similar to that in which we move our arms, they have an immediate and natural perception of the direction they give to their eyes, as we have of the direction we give to our arms; and perceive the situation of visible objects by their eyes, in a manner similar to that in which we perceive the situ- ation of tangible objects with our hands.
We cannot teach brute animals to use their eyes in any other way than in that which nature hath taught them; nor can we teach them to communicate to us the appearances which visible objects make to them, either in ordinary or in extraordinary cases. We have not therefore the same means of discovering the laws of vision in them as in our own kind, but must satisfy ourselves with probable con- jectures: and what we have said upon this subject is chiefly intended to show that animals, to which nature hath given eyes differing in their number, in their position, and in their natural motions, may very probably be sub- jected to different laws of vision, adapted to the peculiari- ties of their organs of vision.
164 OF THE HUMAN MIND. [CHAP. VI.
SECT. XV.
Squinting considered hypotheticalty.
WHETHER there be corresponding points in the retinae of those whe have an involuntary squint? and if there are, whether they be situate in the same manner as in those who have no squint? are not questions of mere curiosity. They are of real importance to the physician who attempts the cure of a squint, and to the patient who submits to the cure. After so much has been said of the strabismus, or squint, both by medical and by optical writers, one might expect to find abundance of facts for determining these questions. Yet I confess I have been disappointed in this expectation, after taking some pains both to make observations, and to collect those which have been made by others.
Nor will this appear very strange, if we consider, that, to make the observations which are necessary for deter- mining these questions, knowledge of the principles of op- tics, and of the laws of vision must concur with opportu- nities rarely to be met with.
Of those who squint, the far greater part have no dis- tinct vision with one eye. When this is the case, it is impossible, and indeed of no importance, to determine the situation of the corresponding points. When both eyes are good, they commonly differ so much in their direction, that the same object cannot be seen by both at the same time; and in this case it will be very difficult to determine the situation of the corresponding points; for such persons will probably attend only to the objects of one eye, and the objects of the other will be as little regarded as if they were not seen.
We have before observed, that when we look at a near object and attend to it, we do not perceive the double ap- pearance of more distant objects, even when they are in the same direction and are presented to the eye at the same time. It is probable that a squinting person, when SECT. XV.3 OF SEEING. 165 he attends to the objects of one eye, will, in like manner, have his attention totally diverted from the objects of the other; and that he will perceive them as little as we per- ceive the double appearances of objects when we use our eyes in the natural way. Such a person, therefore, unless he is so much a philosopher as to have acquired the habit of attending very accurately to the visible appearances of objects, and even of objects which he does not look at, will not be able to give any light to the questions now under consideration.
It is very probable that hares, rabbits, birds, and fishes, whose eyes are fixed in an adverse position, have the na- tural faculty of attending at the same time to visible objects placed in different, and even in contrary directions; be- cause, without this faculty, they could not have those advantages from the contrary direction of their eyes, which nature seems to have intended. But it is not probable that those who squint have any such natural faculty; be- cause we find no such faculty in the rest of the species. We naturally attend to objects placed in the point where the axes of the two eyes meet, and to them only. To give attention to an object in a different direction is unnatural, and not to be learned without pains and practice.
A very convincing proof of this may be drawn from a fact now well known to philosophers: when one eye is shut, there is a certain space within the field of vision, where we can see nothing at all; the space which is di- rectly opposed to that part of the bottom of the eye where the optic nerve enters. This defect of sight, in one part of the eye, is common to all human eyes, and hath been so from the beginning of the world; yet it was never known until the sagacity of the Abb6 Mariotte discovered it in the last century. And now when it is known, it can- not be perceived, but by means of some particular expe- riments, which require care and attention to make them succeed.
What is the reason that so remarkable a defect of sight, so common to all mankind, was so long unknown, and is now perceived with so much difficulty? It is surely this, 166 OF THE HUMAN MIND. [CHAP. VI.
That the defect is at some distance from the axis of the eye, and consequently in a part of the field of vision to which we never attend naturally, and to which we cannot attend at all without the aid of some particular circumstances.
From what we have said, it appears, that, to determine the situation of the corresponding points in the eyes of those who squint is impossible if they do not see distinctly with both eyes; and that it will be very difficult unless the two eyes differ so little in their direction that the same object may be seen with both at the same time. Such patients I apprehend are rare; at least there are veiy few of them with whom I have had the fortune to meet: and therefore, for the assistance of those who may have hap- pier opportunities and inclination to make the proper use of them, we shall consider the case of squinting hypothc- tically, pointing out the proper articles of inquiry, the ob- servations that are wanted, and the conclusions that may be drawn from them.
1. It ought to be inquired, Whether the squinting per- son sees equally well with both eyes? and, if there be a defect in one, the nature and degree of that defect ought to be remarked. The experiments by which this may be done are so obvious that I need not mention them. But I would advise the observer to make the proper experi- ments, and not to rely upon the testimony of the patient; because I have found many instances, both of persons that squinted and others, who were found, upon trial, to have a great defect in the sight of one eye, although they were never aware of it before. In all the following articles, it is supposed that the patient sees with both eyes so well as to be able to read with either when the other is covered.
2. It ought to be inquired, Whether, when one eye is covered, the other is turned directly to the object? This ought to be tried in both eyes successively. By this observation, as a touchstone, we may try the hypothesis concerning squinting, invented by M. de la Hire, and adopted by Boerhaave, and many others of the medical faculty.
The hypothesis is, That in one eye of a squinting per- SECT. XV.] OF SEEING. 167 son, the greatest sensibility and the most distinct vision is not, as in other men, in the centre of the retina, but upon one side of the centre; and that he turns the axis of this eye aside from the object, in order that the picture of the object may fall upon the most sensible part of the retina, and thereby give the most distinct vision. If this is the cause of squinting, the squinting eye will be turned aside from the object, when the other eye is covered, as well as when it is not.
A trial so easy to be made never was made for more than forty years; but the hypothesis was very generally received. So prone are men to invent hypotheses, and so backward to examine them by facts. At last Dr. Jurin, having made the trial, found that persons who squint turn the axis of the squinting eye directly to the object, when the other eye is covered. This fact is confirmed by Dr. Porterfield; and I have found it verified in all the instances that have fallen under my observation.
3. It ought to be inquired, Whether the axes of the two eyes follow one another, so as to have always the same inclination, or make the same angle, when the person looks to the right or to the left, upward or downward, or straight forward. By this observation we may judge, whether a squint is owing to any defect in the muscles which move the eye, as some have supposed. In the fol- lowing articles we suppose that the inclination of the axes of the eyes is found to be always the same.
4. It ought to be inquired, Whether the person that squints sees an object single or double?
If he sees the object double; and if the two appear- ances have an angular distance equal to the angle which the axes of his eyes make with each other, it may be con- cluded that he hath corresponding points in the retince of his eyes, and that they have the same situations as in those who have no squint. If the two appearances should have an angular distance which is always the same, but manifestly greater or less than the angle contained under the optic axes, this would indicate corresponding points in the retina-, whose situation is not the same as in those who 168 OF THE HUMAN MIND. [CHAP. VI have no squint; but it is difficult to judge accurately the angle which the optic axes make.
A squint too small to be perceived may occasion double vision of objects: for if we speak strictly, every persi squints more or less, whose optic axes do not meet exact in the object which he looks at. Thus, if a man can on bring the axes of his eyes to be parallel, but cannot mal them converge in the least, he must have a small squint looking at near objects, and will see them double, while sees very distant objects single. Again, if the optic ax always converge, so as to meet eight or ten feet before t face at farthest, such person will see near objects singl. but when he looks at very distant objects, he will squin little and see them double.
An instance of this kind is related by Aguilonius in 1 Optics; who says, that he had seen a young man to wh< near objects appeared single, but distant objects appeal double.
Dr. Briggs, in his Nova Visionis Theoria, having c lected from authors several instances of double visit quotes this from Aguilonius, as the most wonderful a unaccountable of all, in so much that he suspects so imposition on the part of the young man: but to th< who understand the laws by which single and double visi are regulated, it appears to be the natural effect of a \< small squint.
Double vision may always be owing to a small squi when the two appearances are seen at a small angular * tance, although no squint was observed: and I do not member any instances of double vision recorded by authc wherein any account is given of the angular distance of appearances.
In almost all the instances of double vision, there reason to suspect a squint or distortion of the eyes, fr the concomitant circumstances, which we find to be on^ or other of the following: the approach of death, or of n deliquium, excessive drinking or other intemperance, vio- lent headach, blistering the head, smoking tobacco, bio or wounds in the head. In all these cases it is reas< SECT. XV.] OF SEEING. 169 to suspect a distortion of the eyes, either from spasm or paralysis in the muscles that move them. But although it be probable that there is always a squint, greater or less, where there is double vision; yet it is certain that there is not double vision always where there is a squint. I know no instance of double vision that continued for life, or even for a great number of years. We shall therefore suppose, in the following articles, that the squinting person sees ob- jects single.
5. The next inquiry then ought to be, Whether the ob- ject is seen with both eyes at the same time, or only with the eye whose axis is directed to it? It hath been taken for granted, by the writers upon the strabismus, before Dr. J 11 rin, that those who squint commonly see objects single with both eyes at the same time; but I know not one fact advanced by any writer which proves it. Dr. Jurin is of a contrary opinion; and as it is of consequence, so it is very easy to determine this point in particular instances, by this obvious experiment. While the person that squints looks steadily at an object, let the observer carefully re- mark the direction of both his eyes, and observe their mo- tions; and let an opaque body be interposed between the object and the two eyes successively. If the patient, not- withstanding this interposition, and without changing the direction of the eyes, continues to see the object all the time, it may be concluded that he saw it with both eyes at once. But if the interposition of the body between one eye and the object makes it disappear, then we may be certain, that it was seen by that eye only. In the two following articles, we shall suppose the first to happen, according to the common hypothesis.
6. Upon this supposition it ought to be inquired, Whether the patient sees an object double in those circumstances wherein it appears double to them who have no squint? Let him, for instance, place a candle at the distance of ten feet; and holding his finger at arm's length between him and the candle, let him observe, when he looks at the candle, whether he 'sees his finger with both eyes, and whether he sees it single or double; and when he looks at I 170 OF THE HUMAN MIND. [CHAP. VI.
his finger, let him observe whether he sees the candle with both eyes, and whether single or double.
By this observation it may be determined, whether to this patient the phenomena of double as well as of single vision are the same as to them who have no squint. If they are not the same; if he sees objects single with two eyes, not only in the cases wherein they appear single, but in those also wherein they appear double to other men; the conclusion to be drawn from this supposition is, that his single vision does not arise from corresponding points in the retina of his eyes; and that the laws of vision are not the same in him as in the rest of mankind.