When we hear an unusual sound, the sensation indeed is in the mind, but we know that there is something ex- ternal that produced this sound. At the same time, our hearing does not inform us, whether the sounding body is near or at a distance, in this direction or that; and there- fore we look round to discover it.
If any new phenomenon appears in the heavens, we sec exactly its colour, its apparent place, magnitude, and figure, but we see not its distance. It may be in the at- mosphere, it may be among the planets, or it may be ii< the sphere of the fixed stars, for any thing the eye can de- termine.
The testimony of the sense of touch reaches only to ob- jects that are contiguous to the organ, but with regard to them, is more precise and determinate. When we feel a body with our hand, we know the figure, distance, and po- sition of it, as well as whether it is rough or smooth, hard or soft, hot or cold.
The sensations of touch, of seeing, and hearing, are all in the mind, and can have no existence but when they are perceived. How do they all constantly and invariably suggest the conception and belief of external objects, which exist whether they are perceived or not? No philo- sopher can give any other answer to this, but that such is the constitution of our nature. How do we know, that the object of touch is at the fingers end, and no where else? That the object of sight is in such a direction from the eye, and in no other, but may be at any distance? and that the object of hearing may be at any distance, and in any direction? Not by custom surely; not by reason- ing, or comparing ideas, but by the constitution of our na- ture. How do we perceive visible objects in the direction of right lines perpendicular to that part of the retina on which the rays strike, while we do not perceive the ob- jects of hearing in lines perpendicular to the membrana tympani, upon which the vibrations of the air strike? Be- cause such are the laws of our nature. How do we know the parts of our bodies affected by particular pains? Not by experience or by reasoning, but by the constitution of nature. The sensation of pain is, no doubt, in the mind, SECT. XII.] OF SEEING. 145 and cannot be said to have any relation, from its own na- ture, to any part of the body: but this sensation, by our constitution, gives a perception of some particular part of the body, whose disorder causes the uneasy sensation. If it were not so, a man who never before felt either the gout or the toothach, when he is first seized with the gout in his toe, might mistake it for the toothach.
Every sense, therefore, hath its peculiar laws and limits, by the constitution of our nature: and one of the laws of siffht is, that we always see an object in the direction of the right line passing from its image on the retina through the centre of the eye.
3. Perhaps some readers will imagine, that it is easier, and will answer the purpose as well, to conceive a law of nature, by which we shall always see objects in the place in which they are, and in their true position, without having recourse to images on the retina, or to the optical centre of the eye.
To this I answer, that nothing can be a law of nature which is contrary to fact. The laws of nature are the most general facts we can discover in the operations of nature. Like other facts, they are not to be hit upon by a happy conjecture, but justly deduced from observation: Like other general facts, they are not to be drawn from a few particulars, but from a copious, patient, and cautious induction. That we see things always in their true place and position is not fact; and therefore it can be no law of nature. In a plain mirror, I see myself, and other things, in places very different from those they really occupy. And so it happens in every instance, wherein the rays coming from the object are either reflected or refracted before falling upon the eye. Those who know any thing of optics know that, in all such cases, the object is seen in the direction of a line passing from the centre of the eye to the point where the rays were last reflected or refracted; and that upon this all the powers of the telescope and mi- croscope depend.
Shall we say, then, that it is a law of nature, that the H 146 OF THE HUMAN MIND. [CHAP. VI.
object is seen in the direction which the rays have when they fall on the eye, or rather in the direction contrary to that of the rays when they fall upon the eye? No. This is not true, and therefore it is no law of nature. For the rays, from any one point of the object, come to all parts of the pupil; and therefore must have different directions: but we see the object only in one of these directions, to wit, in the direction of the rays that come to the centre of the eye. And this holds true, even when the rays that should pass through the centre are stopped, and the object is seen by rays that pass at a distance from the centre.
Perhaps it may still be imagined, that although we are not made so as to see objects always in their true place, nor so as to see them precisely in the direction of the rays when they fall upon the cornea; yet we may be so made as to see the object in the direction which the rays have when they fall upon the retina, after they have undergone all their refractions in the eye, that is, in the direction in which the rays pass from the crystalline to the retina. But neither is this true; and consequently it is no law of our constitution. In order to see that it is not true, we must conceive all the rays that pass from the crystalline to one point of the retina, as forming a small cone, whose base is upon the back of the crystalline, and whose vertex is a point of the retina. It is evident, that the rays which form the picture in this point have various directions, even after they pass the crystalline; yet the object is seen only in one of these directions, to wit, hi the direction of the rays that come from the centre of the eye. Nor is this owing to any particular virtue in the central rays, or in the centre itself; for the central rays may be stopped. When they are stopped, the image will be formed upon the same point of the retina as before, by rays that are not central, nor have the same direction which the central rays had: and, in this case, the object is seen in the same direction as before, although there are now no rays coming in that direction.
From this induction we conclude, That our seeing an ob- SECT. XII.] OF SEEING. 147 ject in that particular direction in which we do see it is not owing to any law of nature by which we are made to see it in the direction of the rays, either before their refractions in the eye, or after, but to a law of our nature, by which we see the object in the direction of the right line that passeth from the picture of the object upon the retina to the centre of the eye.
The facts upon which I ground this induction are taken from some curious experiments of Scheiner, in his Funda- ment. Optic, quoted by Dr. Porterfield, and confirmed by his experience. I have also repeated these experi- ments, and found them to answer. As they are easily made, and tend to illustrate and confirm the law of nature I have mentioned, I shall recite them as briefly and dis- tinctly as I can.
Experiment 1. Let a very small object, such as the head of a pin, well illuminated, be fixed at such a distance from the eye, as to be beyond the nearest limit, and within the farthest limit of distinct vision: For a young eye, not near-sighted, the object may be placed at the distance of eighteen inches. Let the eye be kept steadily in one place, and take a distinct view of the object. We know from the principles of optics, that the rays from any one point of this object, whether they pass through the centre of the eye, or at any distance from the centre which the breadth of the pupil will permit, do all unite again in one point of the retina. We know also, that these rays have different directions, both before they fall upon the eye, and after they pass through the crystalline.
Now, we can see the object by any one small parcel of these rays, excluding the rest, by looking through a small pin-hole in a card. Moving this pin-hole over the various parts of the pupil, we can see the object, first by the rays that pass above the centre of the eye, then by the central rays, then by the rays that pass below the centre, and in like manner by the rays that pass on the right and left of the centre. Thus, we view this object successively, by rays that are central, and by rays that are not central; by rays that have different directions, and are variously in- H2 148 OF THE HUMAN MIND. [cHAP. VI.
clined to each other, both when they fall upon the cornea, and when they fall upon the retina; but always by rays which fall upon the same point of the retina. And what is the event? It is this, that the object is seen in the same individual direction, whether seen by all these rays together, or by any one parcel of them.
Experiment 2. Let the object above mentioned be now placed within the nearest limit of distinct vision, that is, for an eye that is not near-sighted, at the distance of four or five inches. We know, that in this case, the rays coming from one point of the object, do not meet in one point of the retina, but spread over a small circular spot of it; the central rays occupying the centre of this circle, the rays that pass above the centre occupying the upper part of the circular spot, and so of the rest. And we know that the object is in this case seen confused, eveiy point of it being seen, not in one, but in various directions. To remedy this confusion, we look at the object through the pin hole, and while we move the pin-hole over the various parts of the pupil, the object does not keep its place, but seems to move in a contrary direction.
It is here to be observed, that when the pin-hole is car- ried upwards over the pupil, the picture of the object is carried upwards upon the retina, and the object at the same time seems to move downwards, so as to be always in the right line passing from the picture through the centre of the eye. It is likewise to be observed, that the rays which form the upper and the lower pictures upon the retina, do not cross each other as in ordinary vision; yet still the higher picture shows the object lower, and the lower picture shows the object higher, in the same manner as when the rays cross each other. Whence we may observe, by the way, that this phenomenon of our seeing objects in a position contrary to that of their pictures upon the retina does not depend upon the crossing of the rays, as Kepler and Des Cartes conceived.
Experiment 3. Other things remaining as in the last experiment; make three pin-holes in a straight line, so near that the rays coming from the object through all the holes SECT. XII.] OF SEEING. 149 may enter the pupil at the same time. In this case we have a very curious phenomenon; for the object is seen triple with one eye. And if you make more holes within the breadth of the pupil, you will see as many objects as there are holes. However, we shall suppose them only three; one on the right, one in the middle, and one on the left; in which case, you see three objects standing in a line from right to left.
It is here to be observed, that there are three pictures on the retina; that on the left being formed by the rays which pass on the left of the eye's centre; the middle picture being formed by the central rays, and the right hand picture by the rays which pass on the right of the eye's centre. It is farther to be observed, that the object which appears on the right is not that which is seen through the hole on the right, but that which is seen through the hole on the left; and in like manner, the left hand object is seen through the hole on the right, as is easily proved by covering the holes successively. So that whatever is the direction of the rays which form the right hand and left hand pictures, still the right hand pic- ture shows a left hand object, and the left hand picture shows a right hand object.
Experiment 4. It is easy to see how the two last expe- riments may be varied, by placing the object beyond the farthest limit of distinct vision. In order to make this experiment, I looked at a candle at the distance of ten feet, and put the eye of my spectacles behind the card, that the rays from the same point of the object might meet, and cross each other before they reached the retina. In this case, as in the former, the candle was seen triple through the three pin-holes; but the candle on the right was seen through the hole on the right; and, on the con- trary, the left hand candle was seen through the hole on the left. In this experiment, it is evident from the princi- ples of optics, that the rays forming the several pictures on the retina cross each other a little before they reach the retina; and therefore the left hand picture is formed by the rays which pass through the hole on the right; so that 150 OF THE HUMAN MIND. [CHAP. Vf* the position of the pictures is contrary to that of the holes by which they are formed; and therefore is also contrary to that of their objects, as we have found it to be in the former experiments.
These experiments exhibit several uncommon pheno- mena, that regard the apparent place, and the direction of visible objects from the eye; phenomena that seem to be most contrary to the common rules of vision. When we look at the same time through three holes that are in a right line, and at certain distances from each other, we expect that the objects seen through them should really be, and should appear to be, at a distance from each other: Yet, by the first experiment we may, through three such holes, see the same object and the same point of that object; and through all the three it appears in the same individual place and direction.
When the rays of light come from the object in right lines to the eye, without any reflection, inflection, or refraction, we expect that the object should appear in its real and proper direction from the eye; and so it com- monly does: but in the second, third, and fourth experi- ments, we see the object in a direction which is not its true and real direction from the eye, although the rays come from the object to the eye, without any inflection, reflection, or refraction.
When both the object and the eye are fixed without the least motion, and the medium unchanged, we expect that the object should appear to rest, and keep the same place: Yet in the second and fourth experiments, when both the eye and the object are at rest, and the medium unchanged, we make the object appear to move upwards or down- wards, or in any direction we please.
When we look at the same time, and with the same eye, through holes that stand in a line from right to left, we expect that the object seen through the left hand hole should appear on the left, and the object seen through the right hand hole should appear on the right: Yet, in the third experiment, we find the direct contrary.
Although many instances occur in seeing the same SECT. XII.] OF SEEING. 151 object double with two eyes, we always expect that it should appear single when seen only by one eye: Yet in the second and fourth experiments, we have instances wherein the same object may appear double, triple, or quadruple to one eye, without the help of a polyhedron or multiplying glass.
All these extraordinary phenomena, regarding the di- rection of visible objects from the eye, as well as those that are common and ordinary, lead us to that law of nature which I have mentioned, and are the necessary con- sequences of it. And, as there is no probability that we shall ever be able to give a reason why pictures upon the retina make us see external objects any more than pictures upon the hand or upon the cheek; or, that we shall ever be able to give a reason why we see the object in the direction of a line passing from its picture through the centre of the eye, rather than in any other direction: I am therefore apt to look upon this law as a primary law of our constitution.
To prevent being misunderstood, I beg the reader to observe, that I do not mean to affirm, that the picture upon the retina will make us see an object in the direction mentioned, or in any direction, unless the optic nerve and the other more immediate instruments of vision be sound, and perform their function. We know not well what is the office of the optic nerve, nor in what manner it per- forms that office; but that it hath some part in the faculty of seeing seems to be certain; because in an amaurosis, which is believed to be a disorder of the optic nerve, the pictures on the retina are clear and distinct, and yet there is no vision.
We know still less of the use and function of the chorid membrane; but it seems likewise to be necessary to vision: for it is well known, that pictures upon that part of the retina where it is not covered by the chorid, I mean at the entrance of the optic nerve, produce no vision, any more than a picture upon the hand. We acknow- ledge, therefore, that the retina is not the last and most immediate instrument of the mind in vision. There are 152 OF THE HUMAN MIND. [CHAP. VI.
other material organs, whose operation is necessary to seeing, even after the pictures upon the retina are formed. If ever we come to know the structure and use of the chorid membrane, the optic nerve, and the brain, and what impressions are made upon them by means of the pictures on the retina, some more links of the chain may be brought within our view, and a more general law of vision discovered: but while we know so little of the nature and office of these more immediate instruments of vision, it seems to be impossible to trace its laws beyond the pictures upon the retina.
Neither do I pretend to say, that there may not be dis- eases of the eye, or accidents, which may occasion our seeing objects in a direction somewhat different from that mentioned above. I shall beg leave to mention one in- stance of this kind that concerns myself.
In May, 1761, being occupied in making an exact meridian, in order to observe the transit of Venus, I rashly directed to the sun, by my right eye, the cross hairs of a small telescope. I had often done the like in my younger days with impunity; but I suffered by it at last, which I mention as a warning to others.
I soon observed a remarkable dimness in that eye; and for many weeks, when I was in the dark, or shut my eyes, there appeared before the right eye a lucid spot, which trembled much like the image of the sun seen by reflection from water. This appearance grew fainter and less fre- quent by degrees; so that now there are seldom any remains of it. But some other very sensible effects of this hurt still remain. For, first, The sight of the right eye continues to be more dim than that of the left. Se- condly, The nearest limit of distinct vision is more remote in the right eye than in the other; although, before the time mentioned, they were equal in both these respects, as I had found by many trials. But, thirdly, what I chiefly intended to mention, is, That a straight line, in some cir- cumstances, appears to the right eye to have a curvature in it. Thus, when I look upon a music book, and, shut- ting up my left eye, direct the right to a point of the middle SECT. XII.] OP SEEING.
line of the five which compose the staff of music; the mid- dle 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 out- wards, 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 con- jecture 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 cor- responds, not to the interval of the parts in their present preternatural contraction, but to their interval in their na- tural and sound state.
SECT. XIII.
Of seeing Objects single icith two Eyes.
ANOTHER phenomenon of vision which deserves atten- tion is our seeing objects single with two eyes. There are two pictures 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 phenomenon, given by anatomists and philosophers, seem to be unsatisfactory. I shall pass over tjhe opinions 154 OF THE HUMAN MIND. [CHAP. VI.
of Galen, of Gassendus, of Baptista Porta, and of Ro- hault. 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 phenomena 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 jany tine judgment or just taste in inquiries of this nature, but is very often overlooked in practice, namely, That in the solution of natural phenomena, all the length that the hu- man faculties can carry us is only this, that from par- ticular phenomena we may, by induction, trace out gene- ral phenomena, of which all the particular ones are neces- sary consequences. And when we have arrived at the most general phenomena 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 particular phenomenon 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 phenomena, but that all bodies whatsoever gravitate towards each other. This is resolving a general phenomenon 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 phenomenon still more general, and of which the gravitation of all bodies is a particular instance. The most general phenomena 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 phenomenon, which human industry 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; SECT. XIII.] OF SEEING. 15o and the less general is contained and comprehended in it. Having premised these things, we proceed to consider the phenomena of single and double vision, in order to dis- cover some general principle to which they all lead, and of which they are the necessary consequences. 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 show the object single are in the centres of the retina. When two pic- tures of a small object are formed upon points of the retina, if they show the object single, we shall, for the sake of perspicuity, call such two points of the retina, corres- ponding 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 phenomenon, it is evident that the two centres of the retina are corresponding points.
2. Supposing the same things as in the last phenome- non; other objects at the same distance from the eyes as that to which their axes are directed do also appear sin- gle. Thus, if I direct my eyes to a candle placed at the distance of ten feet; and, while I look at this candle, an- other stand 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 retina;, 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 demon- strated from the principles of optics. Hence it appear^ that in this second phenomenon of single vision, the cor- responding points are points of the two retince, which are 156 OF THE HUMAN MIND; [CHAP. VI.
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 phenome- non, that every point in one retina corresponds with that which is similarly situate in the other.
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 ringer at arm's length between my eyes and the candle; when I look at the candle, I see my finger double; and when I look at my finger, I 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 phenomenon, it is evident to those who understand the principles of optics, that the pictures of the objects which are seen double do not fall upon points of the retinae which are similarly situate, but that the pictures of the object seen single do fall upon points similarly situate. Whence we infer, that as the points of the two retinae, 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 phenomenon, we have been ac- customed 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.
5. It may however be remarked, that the custom of at- tending to visible appearances has a considerable effect, and makes the phenomenon of double vision to be more or less observed and remembered. Thus 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, put 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, upon the first trial, see the candle double when he looks at his finger; SECT. XIII.] OF SEEING. 157 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 liousand times presented to his eye before now; but he did not attend to it; and so it is as little an object of his •ejection and memory as if it had never happened.