SECTION XVI.

FACTS RELATING TO SQUINTING.

HAVING Considered the phenomena of squinting hypothetically, and their connexion with corresponding points in the retina, 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 possible." 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 direction; 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 last section. He tells us that he had attempted a cure after this method, upon a young gentleman, with promising hopes of success; but was interrupted by his falling ill of the small рох, of

which he died.

It were to be wished that Dr. Jurin had acquainted us, whe ther 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 objects 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 attempt.

But I have lately 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 observed 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 before 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 trials; and when they squint, the best eye is always directed 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 defect in one eye might be the occasion of their squinting, time may discover. The two brothers have found the sight of the weak eye improved 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 toward 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 company 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 together, 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 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 appear 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 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 preternatural 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.

SECTION XVII.

OF THE EFFECT OF CUSTOM IN SEEING OBJECTS SINGLE.

Ir 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 accurately, 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 direction of both eyes to the object, I think it must be acknowledged 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 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 distance only; and at distances much less, or much

greater, would always appear double. The wisdom of nature. is conspicuous in giving us this power, and no less conspicuous 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 inclination 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 constitution is more admirable than that whereby we acquire habits which are found useful without any design or attention. 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 profession and manner of life. A miniature painter, or an engraver, sees very near objects better that 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 distinctly.

It remains to be considered, whether that correspondence between certain points of the retina, which is likewise necessary to single vision, be the effect of custom, 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 correspoding points, the eyes assist each other in vision, and the object is seen better by both together, than it could be by one; but when the eyes are 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 natural 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 in the 11th section, we 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 his 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 must 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 therefore 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