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the eyes is evidently intended to occasion the images of the same thing to fall upon corresponding parts of the retina. Now, this consentaneous motion of the eyes, which causes the images of objects to fall upon corresponding portions of the retina, must be produced by the original conformation and adjustment of the organs of vision, the eyes, the optic nerves, the muscles and other appurtenant membranes. What can be the nature of an adjustment which leads to such curious results? We wish to indulge no hypothesis in philosophy, regarding all hypotheses in science as counterfeit coin, whose circulation is prohibited by the strictest laws of investigation; but in a matter in which we shall probably not be able to attain absolute certainty, may not the solution of Newton, when rightly interpreted and clearly understood, be deemed satisfactory? In his 15th query annexed to his optics, he says, "are not the species of objects seen with both eyes, united, where the optic nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and after union going thence into the brain, in the nerve which is on the right side of the head, and the fibres on the left side of both nerves uniting in the same place, and after union going into the brain, in the nerve which is on the left side of the head, and these two nerves meeting in the brain in such a manner that their fibres make but one entire species or picture, half of which on the right side of the sensorium, comes from the right side of both eyes, through the right side of both optic nerves, to the place where the nerves meet, and from thence on the right side of the head into the brain, and the other half on the left side of the sensorium comes, in like manner, from the left side of both eyes? For the optic nerves of such animals as look the same way with both eyes, as men, dogs, sheep, oxen, &c. meet before they come into the brain, but the optic nerves of such animals as do not look the same way with both eyes, as of fishes and the cameleon, do not meet, if I am rightly informed." I de
not stop here to inquire into the correctness of the opinion, that the optic nerves of such animals, as look the same way with both eyes, meet on their way to the common sensorium, as this is a subject of doubt and disputation. It is certain that they tend towards each other, and very nearly meet in their passage from the eyes, which is all that is necessary to the proof of our theory. Nor do I stop to discuss the incomprehensible doctrine of sensible species, which Newton merely takes up from the schools, in order to convey his ideas. I conceive that when the language of Newton is divested of the scholastic jargon, he will be found to be aiming at the truth. In simple and intelligible phrase, then, the theory which Newton here suggests, and which according to his usual caution, he puts into the form of a mere query, as he was in the habit of doing when he was not able from the facts to arrive at entire certainty, is this: that that adjustment of the optic nerves by which they are made gradually to approach each other, and at length either to meet or nearly to meet, seems like a contrivance to enable us with both eyes to see an object single at the same time. Now is not this in a high degree probable? It is certain, that this is a matter about which it is extremely difficult to obtain conclusive evidence. From the position and delicacy of these organs, experiments cannot be made upon them by the investigator of nature; and of course, we must wait, with patience, for the disclosure of those facts, with which nature herself shall supply us in the ordinary course of providence. But in a case about which, we admit, we are unable to arrive at entire proof, could any final cause seem to be more clearly revealed, than that the optic nerves leading from the eyes to the brain are made to join or approach each other, in order that the actions which are excited in them by the images formed upon the bottom of the eyes, may coalesce into one at their place of junction, and thus occasion in the mind one single perception or impression? Of what nature
that action is, which is excited in the tunica retina or optic nerves, we know not; but that there is an action of some kind or other, has before been shown to be admitted by all philosophers. Now we could not conceive of a contrivance to make these actions upon the nerves of both eyes to commingle in their passage, and become blended into one, and by this means occasion a single impression upon the brain, better calculated for the purpose, than that of causing these fine capillaments to meet, or nearly meet, and touch each other on their way to the common sensorium. We have before stated our views of the system of Hartley, and repudiated it as a mere hypothesis, and unworthy of the philosophy of the present day. A theory, however, which will not bear the test of critical examination, may furnish us with an apt illustration of our doctrine. Suppose these optic nerves, leading from the fund of the eyes to the brain, to be fine elastic cords, like those of a stringed instrument. By being united or nearly united at any given place on their passage to the brain, would not any vibrations excited in them, be so mingled and confounded at their place of junction, as to appear beyond that point to be one single vibration? Why, then, may we not suppose with Newton, that in a mode similar to this, nature, by this arrangement of the optic nerves, enables us to see objects single with both eyes? In this adjustment too, if it be really the product of nature, and not falsely ascribed to her, we may discover a very curious adaptation of this part of the system to another; viz. those muscles appertaining to both eyes by whose means we are able to move them in concert, or preserve their axes parallel to each other, without which, it is well ascertained from fact and experience, we should see every object double. As by the use of these muscles we are enabled to direct both our eyes to a single object, so that the images of it shall fall upon corresponding points of the retina; so, by this tendency of the optic nerves towards each other, one single impression is conveyed to the seat of
sensation. Thus one part of the system is seen ministering to the successful operation of another. The human frame, throughout its whole structure, is admitted to be one of the most curious and wonderful pieces of mechanism that can be presented to our inspection. However distinct the mind may be from the body in its properties and powers, it is certainly dependent upon it in its present state, for many of its most familiar and important operations. If the muscles or tendons refuse to perform their offices, all voluntary motions by their means are suspended. If the parts of the ear contrived for the reception and conveyance of sound be injured or destroyed, there is either no sensation, or a very indistinct perception of sounding bodies. If the humours or lenses of the eye be obscured by accident or disease, or the retina and nerves leading to the brain be disordered, there is no vision. In all these and numberless other cases we find mechanical contrivances rendered subsidiary to the operations of the mind. Why may they not be so in the case under consideration? It seems scarcely more certain, that most of our muscular actions are performed by the operation of muscles and antagonist muscles, or that motions of the bones are performed by means of the hinge joints in some instances, and in others by means of those of the ball and socket, than that the tendency of the optic nerves to unite in their passage from the eyes to the brain, is intended to enable us to see objects single with two eyes.
If the theory above maintained be true, we can readily give a solution of the question with which we commenced this essay. A man born blind, and suddenly restored to sight by the surgeon, upon opening his eyes would not originally see objects double but single, as he does afterwards; and our seeing objects single with two eyes is not an acquired but original perception. This conclusion is confirmed by experience. Cheselden could not discover in any one instance, among those whom he couched, that when
restored to sight they saw objects double; and it is evident that children upon first opening their eyes see objects single, since they move them in perfect concert, and do not allow them to roll in their heads, or wander in different directions. The facility with which we move our eyes from place to place, and object to object, is the result of habit, since neither children, nor those who have been couched, are able to do this at first with facility; but the power of moving both our eyes in correspondence with each other seems to be instinctive.
From this view of the subject, the causes, or most of the causes which occasion double vision, stand clearly revealed. If from any circumstances, as for instance, spasm, disease, accident, extreme debility of the system, alterations produced in the humours and lenses of the eye, or from any other cause of a similar nature, which the experience of physicians only are the most likely to disclose, the organs of vision be impaired, so that the parallel motion of the eyes is interrupted, or the action conveyed by the optick nerves be intercepted or disturbed in its transmission to the brain, or images of objects be formed upon different parts of the retina, the object may appear double. To mention a very ordinary phenomenon, an instance of which came under my own observation. A lady of my acquaintance, on one occasion after the birth of a child, during the heat of summer, became so extremely weak from indisposition, that every object appeared double. That this effect was produced by extreme debility alone, appeared evident, from the circumstance, that as soon as by proper regimen she was restored to her health and strength, the effec ceased. Whether in this case her double vision arose from a weakness in the muscles alone by which the parallel motion of the eyes is effected, or in some diseased or imperfect action in the fine capillaments of the optick nerves, it is impossible to decide. She remarked, however, during the continuance of this