Nor will this appear very strange, if we consider, that to make the observations which are necessary for determining these questions, knowledge of the principles of optics, and of the laws of vision, must concur with opportunities rarely to be met with. Of those who squint, the far greater part have no distinct 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 appearances 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 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 perceive 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 natural faculty of attending at the same time to visible objects placed in different, and even in contrary directions; because, 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; because 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. *On this imperfection of vision is rested the theory of Squinting, proposed by Bufon, and now generally adopted. The defective eye is turned aside, because, if it were directed to the object, together with the perfect one, a confused impression would be produced.-H. 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 directly 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 Abbé Mariotte discovered it in the last century. And now when it is known, it cannot be perceived, but by means of some particular experiments, which require care and attention to make them succeed. What is the reason that so remarkable a defect of sight, common to all mankind, was so long unknown, and is now perceived with so much difficulty? It is surely thisThat 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 par ticular 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 very few of them with whom I have had the fortune to meet : and therefore, for the assistance of those who may have happier opportunities, and inclination to make the proper use of them, we shall consider the case of squinting, hypothetically, pointing out the proper articles of inquiry, the observations that are wanted, and the conclusions that may be drawn from them. 1. It ought to be inquired, Whether the squinting person 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 would advise the observer to make the proper experiments, 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 The hypothesis is, That, in one eye of A trial so easy to be made, never was made for more than forty years; but the hypothesis was very generally receivedso 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 following 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 appearances have an angular distance, equal to the angle which the axes of his eyes make with each other, it may be concluded that he hath corresponding points in the retine of his eyes, and that they have the same situation 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 retine, whose situation is not the same as in those who have no squint; but it is difficult to judge accurately of the angle which the optic axes make. speak strictly, every person squints more or less, whose optic axes do not meet exactly in the object which he looks at. Thus, if a man can only bring the axes of his eyes to be parallel, but cannot make them converge in the least, he must have a small squint in looking at near objects, and will see them double, while he sees very distant objects single. Again, if the optic axes always converge, so as to meet eight or ten feet before the face at farthest, such a person will see near objects single; but when he looks at very distant objects, he will squint a little, and see them double. An instance of this kind is related by Aguilonius in his " Optics," who says, that he had seen a young man to whom near objects appeared single, but distant objects appeared double. Dr Briggs, in his "Nova Visionis Theoria," having collected from authors several instances of double vision, quotes this from Aguilonius, as the most wonderful and unaccountable of all, insomuch that he suspects some imposition on the part of the young man: but to those who understand the laws by which single and double vision are regulated, it appears to be the natural effect of a very small squint." Double vision may always be owing to a small squint, when the two appearances are seen at a small angular distance, although no squint was observed: and I do not remember any instances of double vision recorded by authors, wherein any account is given of the angular distance of the appearances. In almost all the instances of double vision, there is reason to suspect a squint or distortion of the eyes, from the concomitant circumstances, which we find to be one or other of the following-the approach of death or of a deliquium, excessive drinking or other intemperance, violent headache, blistering the head, smoking tobacco, blows or wounds in the head. In all these cases, it is reasonable 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 objects single. 5. The next inquiry, then, ought to be, Whether the object is seen with both eyes at the same time, or only with the eye It is observed by Purkinje and Volkmann, that short-sighted persons, under certain conditions, see Is the case of Aguilonius more than an example of this ?.-H. A squint.too small to be perceived, may distant objects double. occasion double vision of objects: for, if we 1 | 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. whose axis is directed to it? It hath been taken for granted, by the writers upon the strabismus, before Dr Jurin, 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 remark the direction of both his eyes, and observe their motions; and let an opaque body be interposed between the object and the two eyes successively. If the patient, notwithstanding this interposition, and without changing the direction of his 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 If it is true, as has been commonly afto happen, according to the common hypo-firmed, that one who squints sees an object thesis. 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 his finger, let him observe whether he sees the candle with both eyes, and whether single or double. Having premised these definitions, it wil be evident to those who understand the principles of optics, that in this person the natural centre of one retina corresponds with the real centre of the other, in the very 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 renna, and the natural centre in the other, do likewise correspond, in the very same manner as the points similarly situate with regard to the two real centres correspond in perfect eyes. 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 everything double which he saw with both eyes at the same time; and objects distant 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 By this observation, it may be deter-looking straight, than that of another man mined, whether to this patient, the phænomena 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 retine of his eyes; and that the laws of vision are not the same in him as in the rest of mankind. 7. If, on the other hand, he sees objects double in those cases wherein they appear double to others, the conclusion must be, that he hath corresponding points in the retine 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 by squinting. He can never see perfectly when he does not squint, unless the corre sponding 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 tha it is better to put up with the deformity of a squint, than to purchase the cure by the loss of perfect and distinct vision. 8. We shall now return to Dr Jurin's hypothesis, and suppose that our patient, when he saw objects single notwithstanding his quint, was found, upon trial, to have seen them only with one eye. We would advise such a patient to endeavour, by repeated efforts, to lessen his squint, and to bring the axes of his eyes 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 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 considerable degree. Why should it be more difficult for a squinting 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 practice 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 retine, 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 deformity, 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 admonish him when he squints. 10. What is supposed in the 9th article is not merely imaginary; it is really the case of some squinting persons, as will appear in the next section. Therefore, it ought further 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, whether the pupil of the diverging eye is not covered wholly, or in part, by the upper eyelid? Dr Jurin observed instances of these eases in persons that squinted, and assigns them as causes of their seeing the object only with one eye. Thirdly, it may be observed, whether the diverging eye is not so directed, that the picture of the object falls upon that part of the retina where the 11. In the last place, it ought to be inquired, Whether such a person hath any distinct vision at all with the diverging 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 i voluntary 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 everything in his power to remedy it. For alleviating such distresses, nature of en teaches men wonderful expedients, 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 perfection, and become habitual, even without thought or design. Now, in this case, what disturbs the sight of one eye is the sight of the other; and all the disagreeable appearances in vision would cease if the light of one eye was extinct. The sight of one eye will become more distinct 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 acquired 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 be ter than the other; for, in that case, the best eye will always be directed to the object which he intends to look at, and every habit will be acquired which tends to hinder his seeing it 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 increase or diminution of his squint, he may bring it to correspond 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. | 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 satis 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 de-fied that there is nothing peculiar in the termine in the instances that have fallen under my observation-I shall leave to future inquiry. I have endeavoured, in the foregoing articles, to delineate such a process as is proper in observing the phænomena 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 phænomena, 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. Section XVI. FACTS RELATING TO SQUINTING. Having considered the phænomena of squinting, hypothetically, and their connection with corresponding points in the retine, 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. 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 eighth and ninth 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 smallpox, 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 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 ob Dr Porterfield says, that this is generally the case of people that squint : and I suspect it is so more generally than is com-ject, they see it and the adjacent objects monly imagined. Dr Jurin, in a very judicious dissertation upon squinting, printed in Dr Smith's "Optics," observes, that those who squint, and see 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 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 pro |