7. If, on the other hand, he sees objects double in those cases wherein they appear double to others, the conclu- sion must be, that he hath corresponding points in the the retina of his eyes, but unnaturally situate; and their situation may be thus determined.
When he looks at an object, having the axis of one eye directed to it, and the axis of the other turned aside from it; let us suppose a right line to pass from the object through the centre of the diverging eye. We shall, for the sake of perspicuity, call this right line the natural axis of the eye: and it will make an angle with the real axis, greater or less, according as his squint is greater or less. We shall also call that point of the retina in which the natural axis cuts it, the natural centre of the retina; which will be more or less distant from the real centre, according as the squint is greater or less.
Having premised these definitions, it will be evident to those who understand the principles of optics, that in' this person the natural centre of one retina corresponds with theTeal centre of the other, in the veiy same manner as the two real centres correspond in perfect eyes; and that the points similarly situate with regard to the real centre in one retina, and the natural centre in the other, do like- wise correspond in the very same manner as the points sfc milarly situate with regard to the two real centres corre- spond in perfect eyes.
If it is true, as has been commonly affirmed, that one SECT. XV.] OF SEEING. 171 who squints sees an object with both eyes at the same time, and yet sees it single, the squint will most probably be such as we have described in this article. And we may further conclude, that if a person affected with such a squint as we have supposed could be brought to the habit of looking straight, his sight would thereby be greatly hurt. For he would then see every thing double which he saw with both eyes at the same time; and objects dis- tant from one another would appear to be confounded together. His eyes are made for squinting, as much as those of other men are made for looking straight; and his sight would be no less injured by looking straight than that of another man by squinting. He can never see per- fectly when he does not squint, unless the corresponding points of his eyes should by custom change their place; but how small the probability of this is will appear in the 17th section.
Those of the medical faculty who attempt the cure of a squint would do well to consider whether it is attended with such symptoms as are above described. If it is, the cure would be worse than the malady: for every one will readily acknowledge, that it is better to put up with the deformity of a squint than to purchase the cure by the loss of per- fect and distinct vision.
8. We shall now return to Dr. Jurin's hypothesis, and suppose, that our patient when he saw objects single, not- withstanding his squint, was found, upon trial, to have seen them only with one eye.
We should advise such a patient to endeavour, by re- peated efforts, to lessen his squint, and to bring the axes of his eyes nearer to a parallel direction. We have natu- rally the power of making small variations in the inclina- tions of the optic axes; and this power may be greatly in- creased by exercise.
In the ordinary and natural use of our eyes, we can direct their axes to a fixed star; in this case they must be parallel: we can direct them also to an object six inches distant from the eye; and in this case the axes must make 172 OF THE HUMAN MIND. [CHAP. VI.
an angle of fifteen or twenty degrees. We see young people in their frolics learn to squint, making their eyes either converge or diverge, when they will, to a very consider- able degree. Why should it be more difficult for a squint- ing person to learn to look straight when he pleases? If once, by an effort of his will, he can but lessen his squint, frequent practice will make it easy to lessen it, and will daily increase his power. So that if he begins this prac- tice in youth, and perseveres in it, he may probably, after some time, learn to direct both his eyes to one object.
When he hath acquired this power, it will be no difficult matter to determine, by proper observations, whether the centres of the retina;, and other points similarly situate with regard to the centres correspond as in other men.
9. Let us now suppose that he finds this to be the case; and that he sees an object single with both eyes, when the axes of both are directed to it. It will then concern him to acquire the habit of looking straight, as he hath got the power, because he will thereby not only remove a defor- mity, but improve his sight: and I conceive this habit, like all others, may be got by frequent exercise. He may practise before a mirror when alone, and in company he ought to have those about him, who will observe and ad- monish him when he squints.
10. What is supposed in the 9th article is not merely imaginary; it is really the case in some squinting persons, as will appear in the next section. Therefore, it ought farther to be inquired, How it comes to pass, that such a person sees an object which he looks at, only with one eye, when both are open? In order to answer this question it may be observed, first, Whether, when he looks at an object, the diverging eye is not drawn so close to the nose, that it can have no distinct images? Or, secondly, Whe- ther the pupil of the diverging eye is not covered wholly, or in part, by the upper eyelid? Dr. Jurin observed in- stances of these cases in persons that squinted, and as- signs them as causes of their seeing the object only with one eye. Thirdly, it may be observed, Whether the di- SECT. XV.] OF SEEING. 173 verging eye is not so directed that the picture of the object falls upon that part of the retina where the optic nerve enters, and where there is no vision? This will probably happen in a squint wherein the axes of the eyes converge, so as to meet about six inches before the nose.
11. In the last place, it ought to be inquired, Whether such a person hath any distinct vision at all with the di- verging eye, at the time he is looking at an object with the other?
It may seem very improbable, that he should be able to read with the diverging eye when the other is covered, and yet, when both are open, have no distinct vision with it at all. But this perhaps will not appear so improbable, if the following considerations are duly attended to.
Let us suppose that one who saw perfectly gets, by a blow on the head, or some other accident, a permanent and involuntary squint. According to the laws of vision, he will see objects double, and will see objects distant from one another confounded together: but such vision being very disagreeable, as well as inconvenient, he will do every thing in his power to remedy it. For alleviating such dis- tresses, nature often teaches men wonderful experiments, which the sagacity of a philosopher would be unable to discover. Every accidental motion, every direction or conformation of his eyes, which lessens the evil, will be agreeable; it will be repeated until it be learned to per- fection, and become habitual, even without thought or de- sign. Now, in this case, what disturbs the sight of one eye is the sight of the other; and all the disagreeable ap- pearances in vision would cease, if the light of one eye was extinct: the sight of one eye will become more dis- tinct and more agreeable, in the same proportion as that of the other becomes faint and indistinct. It may therefore be expected, that every habit will, by degrees, be ac- quired, which tends to destroy distinct vision in one eye, while it is preserved in the other. These habits will be greatly facilitated, if one eye was at first better than the other; for in that case, the best eye will always be di- 174 OF THE HUMAN MIND. [CHAP, VI.
rected to the object which he intends to look at, and every habit will be acquired which tends to hinder his seeing at all, or seeing it distinctly by the other at the same time.
I shall mention one or two habits that may probably be acquired in such a case; perhaps there are others which we cannot so easily conjecture. First, By a small in- crease or diminution of his squint, he may bring it to cor- respond with one or other of the cases mentioned in the last article. Secondly, The diverging eye may be brought to such a conformation as to be extremely short sighted, and consequently to have no distinct vision of objects at a distance. I knew this to be the case of one person that squinted; but cannot say whether the short sightedness.of the diverging eye was original or acquired by habit.
We see, therefore, that one who squints, and originally saw objects double by reason of that squint, may acquire such habits that, when he looks at an object, he shall see it only with one eye: nay, he may acquire such habits that, when he looks at an object with his best eye, he shall have no distinct vision with the other at all. Whether this is really the case, being unable to determine in the instances that have fallen under my observation, I shall leave to future inquiry.
I have endeavoured, in the foregoing articles, to de- lineate such a process as is proper in observing the phe- nomena of squinting. I know well by experience, that this process appears more easy in theory than it will be found to be in practice; and that, in order to carry it on with success, some qualifications of mind are necessary in the patient, which are not always to be met with. But if those who have proper opportunities and inclination to observe such phenomena attend duly to this process, they may be able to furnish facts less vague and uninstructive than those we meet with, even in authors of reputation. By such facts vain theories may be exploded, and our knowledge of the laws of nature which regard the noblest of our senses enlarged.
OF SEEING. 175 SECT. XVI.
Facts relating to Squinting.
HAVING considered the phenomena of squinting hypo- thetically, and their connexion with corresponding points in the retinae, I shall now mention the facts I have had occasion to observe myself, or have met with in authors, that can give any light to this subject.
Having examined above twenty persons that squinted, I found in all of them a defect in the sight of one eye. Four only had so much of distinct vision in the weak eye as to be able to read with it when the other was covered. The rest saw nothing at all distinctly with one eye.
Dr. Porterfield says, that this is generally the case of people that squint: and I suspect it is so more generally than is commonly imagined. Dr. Jurin, in a very judicious dissertation upon squinting, printed in Dr. Smith's Optics, observes, that those who squint, and see objects with both eyes, never see the same object with both at the same time; that when one eye is directed straight forward to an object, the other is drawn so close to the nose that the object cannot at all be seen by it, the images being too oblique and too indistinct to affect the eye. In some squinting persons he observed the diverging eye drawn under the upper eyelid, while the other was directed to the object. From these observations he concludes, that " the eye is thus distorted, not for the sake of seeing better with it, but rather to avoid seeing at all with it as much' as pos- sible." From all the observations he had made, he was satisfied, that there is nothing peculiar in the structure of a squinting eye; that the fault is only in its wrong direc- tion; and that this wrong direction is got by habit. Therefore he proposes that method of cure which we have described in the 8th and 9th articles of the hist section. He tells us, that he had attempted a cure after this method, upon a young gentleman, with promising hopes of success; 176 OF THE HUMAN MIND. [CHAP. VI.
but was interrupted by his falling ill of the small pox, of which he died.
It were to be wished that Dr. Jurin had acquainted us whether he ever brought the young man to direct the axes of both eyes to the same object, and whether, in that case, he saw the object single, and saw it with both eyes; and that he had likewise acquainted us, whether he saw ob- jects double when his squint was diminished. But as to these facts he is silent.
I wished long for an opportunity of trying Dr. Jurin's method of curing a squint, without finding one; having always, upon examination, discovered so great a defect in the sight of one eye of the patient as discouraged the at- tempt.
But I have already found three young gentlemen, with whom I am hopeful this method may have success, if they have patience, and perseverance in using it. Two of them are brothers, and, before I had access to examine them, had been practising this method by the direction of their tutor, with such success that the elder looks straight when he is upon his guard: the younger can direct both his eyes to one object; but they soon return to their usual squint.
A third young gentleman, who had never heard of this method before, by a few days practice was able to direct both his eyes to one object, but could not keep them long in that direction. All the three agree in this, that when both eyes are directed to one object, they see it and the adjacent objects single; but when they squint they see objects sometimes single and sometimes double. I ob- served of all the three, that when they squinted most, that is, in the way they had been accustomed to, the axes of their eyes converged, so as to meet five or six inches be- fore the nose. It is probable that in this case the picture of the object in the diverging eye must fall upon that part of the retina where the optic nerve enters; and therefore the object could not be seen by that eye.
All the three have some defect in the sight of one eye, which none of them knew until I put them upon making SECT. XVI.] OF SEEING. 177 trials; and when they squint, the best eye is always di- rected to the object, and the weak eye is that which diverges from it. But when the best eye is covered, the weak eye is turned directly to the object. Whether this defect of sight in one eye be the effect of its having been long disused, as it must have been when they squinted; or whether some original defects in one eye might be the occasion of their squinting, time may discover. The two brothers have found the sight of the weak eye even im- proved by using to read with it while the other is covered. The elder can read an ordinary print with the weak eye; the other, as well as the third gentleman, can only read a large print with the weak eye. I have met with one other person only who squinted, and yet could read a large print with the weak eye. He is a young man, whose eyes are both tender and weak sighted, but the left much weaker than the right. When he looks at any object, he always directs the right eye to it, and then the left is turned towards the nose so much that it is impossible for him to see the same object with both eyes at the same time. When the right eye is covered, he turns the left directly to the object; but he sees it indistinctly, and as if it had a mist about it.
I made several experiments, some of them in the com- pany and with the assistance of an ingenious physician, in order to discover whether objects that were in the axes of the two eyes were seen in one place confounded toge- ther, as in those who have no involuntary squint. The object placed in the axis of the weak eye was a lighted candle at the distance of eight or ten feet. Before the other eye was placed a printed book, at such a distance as that he could read upon it. He said, that while he read upon the book, he saw the candle but very faintly. And from what we could learn, these two objects did not ap- pear in one place, but had all that angular distance in appearance which they had in reality.
If this was really the case, the conclusion to be drawn from it is, that the corresponding points in his eyes are not situate in the same manner as in other men; and that if he 178 OF THE HUMAN MIND. [CHAP. VI.
could be brought to direct both eyes to one object, he would see it double. But considering that the young man had never been accustomed to observations of this kind, and that the sight of one eye was so imperfect, I do not pretend to draw this conclusion with certainty from this single instance.
All that can be inferred from these facts is, that of four persons who squint, three appear to have nothing preterna- tural in the structure of their eyes. The centres of the retina, and the points similarly situate with regard to the centres, do certainly correspond in the same manner as in other men. So that if they can be brought to the habit of directing their eyes right to an object, they will not only remove a deformity, but improve their sight. With regard to the fourth, the case is dubious, with some probability of a deviation from the usual course of nature in the situation of the corresponding points of his eyes.
SECT. XVII.
Of the Effect of Custom in seeing Objects Single.
IT appears from the phenomena of single and double vision, recited in Sect. 13, that our seeing an object single with two eyes depends upon these two things — First, Upon that mutual correspondence of certain points of the retina which we have often described. Secondly, Upon the two eyes being directed to the object so accu- rately that the two images of it fall upon corresponding points. These two things must concur in order to our seeing an object single with two eyes; and as far as they depend upon custom, so far only can single vision depend upon custom.
With regard to the second, that is, the accurate direc- tion of both eyes to the object, I think it must be acknow- ledged that this is only learned by custom. Nature hath wisely ordained the eyes to move in such a manner that their axes shall always be nearly parallel; but hath left it SECT. XVII.] OP SEEING. 179 in our power to vary their inclination a little according to the distance of the object we look at. Without this power, objects would appear single at one particular dis- tance only; and, at distances much less or much greater, would always appear double. The wisdom of nature is conspicuous hi giving us this power, and no less conspicu- ous in making the extent of it exactly adequate to the end. The parallelism of the eyes, in general, is therefore the work of nature; but that precise and accurate direction, which must be varied according to the distance of the object, is the effect of custom. The power which nature hath left us of varying the inclination of the optic axes a little is turned into a habit of giving them always that in- clination which is adapted to the distance of the object.
But it may be asked, What gives rise to this habit? The only answer that can be given to this question is, that it is found necessary to perfect and distinct vision. A man who hath lost the sight of one eye very often loses the habit of directing it exactly to the object he looks at, because that habit is no longer of use to him. And if he should recover the sight of his eye, he would recover this habit, by finding it useful. No part of the human consti- tution is more admirable than that whereby we acquire habits which are found useful, without any design or inten- tion. Children must see imperfectly at first; but by using their eyes they learn to use them in the best manner, and acquire, without intending it, the habits necessary for that purpose. Every man becomes most expert in that kind of vision which is most useful to him in his particular profes- sion and manner of life. A miniature painter or an en- graver sees very near objects better than a sailor; but the sailor sees very distant objects much better than they. A person that is short-sighted, in looking at distant objects, gets the habit of contracting the aperture of his eyes, by almost closing his eyelids. Why? For no other reason, but because this makes him see the object more distinct. In like manner, the reason why every man acquires the habit of directing both eyes accurately to the object must be, because thereby he sees it more perfectly and dis- tinctly.
180 OF THE HUMAN MIND. [CHAP. VI.
It remains to be considered, whether that corres- pondence between certain points of the retinae, which is likewise necessary to single vision, be the effect of cus- tom, or an original property of human eyes.
A strong argument for its being an original property may be drawn from the habit just now mentioned, of directing the eyes accurately to an object. This habit is got by our finding it necessary to perfect and distinct vision. But why is it necessary? For no other reason but this, because thereby the two images of the object falling upon corresponding points, the eyes assist each other in vision, and the object is seen better by both to- gether than it could be by one; but when the eyes are not accurately directed, the two images of an object fall upon points that do not correspond, whereby the sight of one eye disturbs the sight of the other, and the object is seen more indistinctly with both eyes than it would be with one. Whence it is reasonable to conclude, that this correspondence of certain points of the retina is prior to the habits we acquire in vision, and consequently is natu- ral and original. We have all acquired the habit of directing our eyes always in a particular manner, which causes single vision. Now, if nature hath ordained that we should have single vision only, when our eyes are thus directed, there is an obvious reason why all mankind should agree in the habit of directing them in this manner.
But if single vision is the effect of custom, any other habit of directing the eyes would have answered the purpose; and no account can be given why this particular habit should be so universal; and it must appear very strange, that no one instance hath been found of a person who had acquired the habit of seeing objects single with both eyes, while they were directed in any other manner.
The judicious Dr. Smith, in his excellent System of Optics, maintains the contrary opinion, and offers some reasonings and facts in proof of it. He agrees with Bishop Berkeley in attributing it entirely to custom, that we see objects single with two eyes, as well as that we see objects erect by inverted images. Having considered Bishop Berkeley's reasonings hi the llth Section, we SECT. XVII.] OF SEEING. 181 shall now beg leave to make some remarks on what Dr. Smith hath said upon this subject, with the respect due to an author to whom the world owes not only many valuable discoveries of his own, but those of the brightest mathematical genius of this age, which, with great labour he generously redeemed from oblivion.
He observes, that the question, Why we see objects single with two eyes? is of the same sort with this, Why we hear sounds single with two ears? and that the same answer must serve both. The inference intended to be drawn from this observation is, that as the second of these phenomena is the effect of custom, so likewise is the first.
Now, I humbly conceive that the questions are not so much of the same sort that the same answer may serve for both; and moreover, that our hearing single with two ears is not the effect of custom.
Two or more visible objects, although perfectly similar, and seen at the very same time, may be distinguished by their visible places; but two sounds perfectly similar, and heard at the same time, cannot be distinguished; for, from the nature of sound, the sensations they occasion must coalesce into one, and lose all distinction. If there- fore it is asked, Why we hear sounds single with two ears? I answer, Not from custom; but because two sounds which are perfectly like and synchronous, have nothing by which they can be distinguished. But will this answer fit the other question? I think not.
The object makes an appearance to each eye, as the sound makes an impression upon each ear: so far the two senses agree. But the visible appearances may be distin- guished by place, when perfectly like in other respects: the sounds cannot be thus distinguished; and herein the two senses differ. Indeed, if the two appearances have the same visible place, they are, in that case, as incapable of distinction as the sounds were, and we see the object single. But when they have not the same visible place, they are perfectly distinguishable, and we see the object double. We see the object single only when the eye* are directed in one particular manner; while there are many 182 OF THE HUMAN MIND. [CHAP. VI.
other ways of directing them within the sphere of our power, by which we see the object double.
Dr. Smith justly attributes to custom that well known fallacy in feeling, whereby a button pressed with two op- posite sides of two contiguous fingers laid across is felt double. I agree with him, that the cause of this appear- ance is, that those opposite sides of the fingers have never been used to feel the same object, but two different ob- jects, at the same time. And I beg leave to add, that as custom produces this phenomenon, so a contrary custom destroys it: for if a man frequently accustoms himself to feel the button with his fingers across, it will at last be felt single; as I have found by experience.