dle line of the five which compose the staff of music, the middle line appears dim, indeed, at the point to which the eye is directed, but straight; at the same time, the two lines above it, and the two below it, appear to be bent outwards, and to be more distant from each other and from the middle line, than at other parts of the staff, to which the eye is not directed. Fourthly, Although I have repeated this experiment times innumerable, within these sixteen months, I do not find that custom and experience takes away this appearance of curvature in straight lines. Lastly, This appearance of curvature is perceptible when I look with the right eye only, but not when I look with both eyes; yet I see better with both eyes together, than even with the left eye alone. I have related this fact minutely as it is, without regard to any hypothesis; because I think such uncommon facts deserve to be recorded. I shall leave it to others to conjecture the cause of this appearance. To me it seems most probable, that a small part of the retina towards the centre is shrunk, and that thereby the contiguous parts are drawn nearer to the centre, and to one another, than they were before; and that objects, whose images fall on these parts, appear at that distance from each other which corresponds, not to the interval of the parts in their present preternatural contraction, but to their interval in their natural and sound state. Section XIII. OF SEEING OBJECTS SINGLE WITH TWO EYES. Another phænomenon of vision which deserves attention, is our seeing objects single with two eyes. There are two pic The opinions relative to single vision with two eves, may, I think, be reduced to two supreme classes The ore attempts to shew that trcre is no difficulty to be solved; the other attempts to solve the difficulty which is admitted. - Under the former class, there are, as I recol ect, three hypotheses. The first - up. poses that we see only with one eye-that man is in Irality a Cyclops; the second suppose that the two imesions are not, in fact, made at the same instant in both eyes and, consequently, that two simulta Frou impressions are not conveyed to the brain and mind; the find supposes that, although a separate impression be made on each retina, yet that the-e seva impress on are, as it were, fused into one before they reach the common sensory, in couse. que ce or a un on of the optic nerves-The hypotesos of the latter class which, I think, may also be redu.ed to three, all admit that there are simultaneous impressions on the two retmæ, aid that these im. prissions are separately conveyed to the termination of the on auie aj paratus; but still hold ha, in the mind, there is cetermined only a single perception, One opinion allows the perception to have been origi. naly world, and saves the phænomenon, by suppos ing that it became sig ethroug the influence of cus. tom and association Another explains it more subjectively, by an ultimate and inexplicable law of our | tures of the object, one on each retina ; and each picture by itself makes us see an object in a certain direction from the eye; yet both together commonly make us see only one object. All the accounts or solutions of this phænomenon given by anatomists and philosophers seem to be unsatisfactory. I shall pass over the opinions of Galen, of Gassendus, of Baptista Porta, and of Rohault. The reader may see these examined and refuted by Dr Porterfield. I shall examine Dr Porterfield's own opinion. Bishop Berkeley's, and some others. But it will be necessary first to ascertain the facts: for, if we mistake the phænomena of single and double vision. it is ten to one but this mistake will lead us wrong in assigning the causes. This likewise we ought carefully to attend to, which is acknowledged in theory by all who have any true judgment or just taste in inquiries of this nature, but is very often overlooked in practice-namely, that, in the solution of natural phænomena, all the length that the human faculties can carry us, is only this, that, from particular phænomena, we may, by induction, trace out general phænomena, of which all the particular ones are necessary consequences. And when we have arrived at the most general phænomena we can reach, there we must stop. If it is asked, Why such a body gravitates towards the earth? all the answer that can be given is, Because all bodies gravitate towards the earth. This is resolving a part cular phænomenon into a general one. If it should again be asked, Why do all bodies gravitate towards the earth? we can give no other solution of this phænomenon, but that all bodies whatsoever gravitate towards each other. This is resolving a general phænomenon into a more general one. If it should be asked, Why all bodies gravitate to one another? we cannot tell; but, if we could tell, it could only be by resolving this universal gravitation of bodies into some other phænomenon still more general, and of which the gravitation of all bodies is a particular instance. The most general phænomena we can reach, are what we call laws of nature; so that the laws of nature are nothing else but the most general facts relating to the operations of nature, which include a great many particular facts under them. And if, in any case, we should give the name of a law of nature to a general phænomenon, which human i dustry shall afterwards trace to one more general, there is no great harm done. The most general assumes the name of a law of nature when it is discovered, and the less general is contained and comprehended in it. Having premised these things, we proceed to consider the phænomena of single constitution; and the last, more objectively, on some intelligible principle of optics.-H. and double vision, in order to discover some general principle to which they all lead, and of which they are the necessary conscquences. If we can discover any such general principle, it must either be a law of nature, or the necessary consequence of some law of nature; and its authority will be equal whether it is the first or the last. 1. We find that, when the eyes are sound and perfect, and the axes of both directed to one point, an object placed in that point is seen single-and here we observe, that in this case the two pictures which shew the object single, are in the centres of the retina. When two pictures of a small object are formed upon points of the retina, if they shew the object single, we shall, for the sake of perspicuity, call such two points of the retina, corresponding points; and where the object is seen double, we shall call the points of the retina on which the pictures are formed, points that do not correspond.* Now, in this first phænomenon, it is evident, that the two centres of the retina are corresponding points. 2. Supposing the same things as in the last phænomenon, other objects at the same distance from the eyes as that to which their axes are directed, do also appear single. Thus, if I direct my eyes to a candle placed at the distance of ten feet, and, while I look at this candle, another stands at the same distance from my eyes, within the field of vision, I can, while I look at the first candle, attend to the appearance which the second makes to the eye; and I find that in this case it always appears single. It is here to be observed, that the pictures of the second candle do not fall upon the centres of the retine, but they both fall upon the same side of the centres-that is, both to the right, or both to the left; and both are at the same distance from the centres. This might easily be demonstrated from the principles of optics. Hence it appears, that in this second phænomenon of single vision, the corresponding points are points of the two retine, which are similarly situate with respect to the two centres, being both upon the same side of the centre, and at the same distance from it. It appears likewise, from this phenomenon, that every point in one retina corresponds with that which is similarly situate in the other. It is to be noticed that Reid uses the terms, cor 3. Supposing still the same things, objects which are much nearer to the eyes, or much more distant from them, than that to which the two eyes are directed, appear double. Thus, if the candle is placed at the distance of ten feet, and I hold my finger at arms-length between my eyes and the candle-when I look at the candle, I see my fin ger double; and when I look at my finger, 1 see the candle double; and the same thing happens with regard to all other objects at like distances which fall within the sphere of vision. In this phænomenon, it is evident to those who understand the prin ciples of optics, that the pictures of the ob jects which are seen double, do not fall upon points of the retine which are similarly situate, but that the pictures of the objects seen single do fall upon points similarly situate. Whence we infer, that, as the points of the two retine, which are similarly situate with regard to the centres, do correspond, so those which are dissimilarly situate do not correspond. 4. It is to be observed, that, although, in such cases as are mentioned in the last phænomenon, we have been accustomed from infancy to see objects double which we know to be single; yet custom, and experience of the unity of the object, never take away this appearance of duplicity. Thus 5. It may, however, be remarked that the custom of attending to visible appearances has a considerable effect, and makes the phænomenon of double vision to be more or less observed and remembered. you may find a man that can say, with a good conscience, that he never saw things double all his life; yet this very man, pat in the situation above mentioned, with his finger between him and the candle, and desired to attend to the appearance of the object which he does not look at, will, upou the first trial, see the candle double, when he looks at his finger; and his finger double, when he looks at the candle. Does he now see otherwise than he saw before? No, surely; but he now attends to what he never attended to before. The same double appearance of an object hath been a thousand times presented to his eye before now, but he did not attend to it; and so it is as little an object of his reflection and memory, as if it had never happened. When we look at an object, the circumjacent objects may be seen at the same time, although more obscurely and indis responding points in a sense opposite to that of tinctly: for the eye hath a considerable Smith, and some optical writers; they use it anatomically, he physiologically. Two points are anatomi. cally correspondent, when on opposite sides of the bedy they severally hold the same relation to the centre. J. Mueller, and other recent physiologists, employ these terms in the same signification as Reid. field of vision, which it takes in at once. But we attend only to the object we look at The other objects which fall within the field of vision, are not attended to; and therefore An argument a priort has been employed against are as if they were not scen. If any of the doctrine here maintained, on the ground that the congruent points in the opposite eyes are not anatomically corresponding points.-H. them draws our attention, it naturally draws the eyes at the same time: for, in the com mon course of life, the eyes always follow | the attention: or if at any time, in a revery, 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 double. When he looks at any object, he sees it single, and takes no notice of other visible objects at that time, whether 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 perhaps he never used, nor attempted; and therefore he does not recollect that ever he saw an object double. But when he is put upon giving this attention, he immediately sees objects 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. apparent distance 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 distance; the finger towards the left being seen by the right eye, and the other by the left. If the head is laid horizontally 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 spectators in one and the same manner. 7. Having made many experiments in order to ascertain the apparent distance of the two appearances of an object seen double, I have found that in all cases this apparent 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 proportioned to the arch of the retina, which lies between their pictures, in like manner, when an object is seen double with the two eyes, the apparent There are many phænomena of a similar distance of the two appearances is propornature, which shew that the mind may not tioned to the arch of either retina, which attend to, and thereby, in some sort, not lies between the picture in that retina, and perceive objects that strike the senses. I the point corresponding to that of the pichad occasion to mention several instances ture in the other retina. of this in 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 angular 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 different persons, who see perfectly with both eyes, shall place their finger 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 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 binocular 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 in the axis of the other eye, we shall see but one shilling. If 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." This last statement is incorrect; it misrepresents. if it does not reverse, the observation of Du Tour. But, though Reid's assertion be inaccurate, there is tution of their organs) in the phænomeno", as regreat difference (proba ly from the different constiported by various observers. None, seemingly, (the reverse of what Reid says,) in looking, e. g., with one eye through a blue, and with the other through a yellow glass, experience a comple mentary sensation of green. But some see both colours at once; some only one colour-a colour, however, which corresponds neither to yellow nor to blue, and, at the same time, is not given. In my This general phænomenon 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, Whether those animals whose eyes have an adverse position in their heads, and look contrary ways, have such corre 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 retina, be natural and original, or the effect of custom? 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? Section XIV. OF THE LAWS OF VISION IN BRUTE ANIMALS. 9. From these phænomena, and from all the trials I have been able to make, it appears evidently, that, in perfect human eyes, the centres of the two renne correspond and harmonize wth 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 in their retine? Secondly, sponding points of the two retina, shew only one object, even when there are really two; and pictures falling upon points of the retina which do not correspond, shew us two visible appearances, although there be but one object: so that pictures, upon corresponding points of the two retina, present the same appearance to the mind as if they had both fallen upon the same point of one retina; and pictures upon points of the two retina, 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 corresponding to it in the other retina. This relation and sympathy between corresponding points of the two retinæ, I do not advance as an hypothesis, but as a general fact or phænomenon of vision. All the phænomena before mentioned, of single or double vision, lead to it, and are necessary 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 phænomena of single and double vision, suppose this general 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," Query, 15. The judicious Dr Smith, in his "Optics," Book 1, § 137, hath confirmed the truth of this general phænomenon from his own experience, not only as to the apparent unity of objects whose pictures fall upon the corresponding points of the retina, but also as to the apparent distance of the two appearances of the same object when seen double. own eye, I can see either of these phænomena, under certain conditions, at will. Johannes Mueller, Wet er, Volkmano, and Heermann, are the most recent observers, I may also notice, that the congruence between the corresponding ponts (in Reid's sense) of he two retinæ, s admitted for the perception of figure, but not for the sensations of Light and colour -H. It might be proper here to say something of the strictures of Dr Wills on Reid's doctrine of single vision; but, as the matter is, after all, of no high psychological importance, while the whole theory of the form of th Ho opter is, in consequence of Mueller's observations, anew under discussion, I shall 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 ordinary cases, every animal, whether it has many eyes or few, whether of one structure or of another, 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 in different animals and insects, 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 pictures 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 which 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 only refer the reader who is curious in such 1oints, to the following recent publications:-J. Mueller, "Zur Vergleichenden Physiologie de Gesichtssinnes," &, 1826 — Volkmann, "Neue Beytraege zur Physiologi des Gesicht sinnes," 183.- Heermann, "Ueber die Bildung der Gesichtsvorstellungen," &c., 1835.-H. sufficient that we be so constituted, that | objects whose pictures are formed upon the centres of the two retine, or upon points similarly situate with regard to these centres, shall be seen in the same visible 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 practice; 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 reta 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 retina which are similarly situate, 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 reasonably think that the case is the same with other animals. But is it not unreasonable to think, that those animals whicli naturally turn one eye towards one object, and another eye towards another object, must thereby have such false appearances presented to then, as we have when we do so against nature? Many animals have their eyes by nature placed adverse and immoveable, the axes of the two eyes being always directed to opposite points. Do objects painted on the centres of the two retine 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 retine in such animals, but of a different kind from that which we have found in human eyes. The centre 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 corre spond with the inferior part of the other, and the anterior part of one with the pos-, terior 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 retine, and points which do not correspond, as the human kind 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 direc tion 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 situation 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 SQUINTING CONSIDERED HYPOTHETICALLY. Whether there be corresponding points in the retine of those who 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. |