NATURAL HISTORY
OF
MAN
SECT. VI.
Of the Sense of Seeing.
We have already described the parts of which the human body consists; and shall now proceed to examine those curious organs by which sensations are conveyed to the mind. In this investigation, we shall endeavour to point out the uses of the different senses, and to mark those errors to which we are, in some measure, subjected by Nature.
In the human foetus, the eyes are early formed; in the chicken also, they are the first double organs which make their appearance; and, in the eggs of lizards, and of several species of birds, I have remarked, that the eyes were more prominent and advanced in growth than any other double parts of the body. In viviparous animals, it is true, and particularly in the human foetus, the eyes are not so large, in proportion, [1] as in the oviparous; but still they are more quickly expanded than the other parts of the system. The same remark applies to the organ of hearing. The small bones of the ear are fully formed before the other bones of the body have acquired any degree of solidity or bulk. In the seventh month, the whole bones of the ear are perfectly solid, and have acquired all the density they possess in the adult state. It is, therefore, apparent, that those parts which are furnished with the greatest quantity of nerves, are first formed and expanded. We formerly remarked, that the vesicles which contain the brain and cerebellum, and that which contains the spinal marrow is a fundamental and essential part of the body, and is therefore first formed. Hence the nerves exist before any of the other parts of the body, and those organs which are most amply supplied with them, as the ears and eyes, are most quickly expanded.
Upon examining the eyes of an infant some hours after birth, it is easy to perceive, that it can make no use of them: This organ not having acquired a sufficient degree of consistence, the rays of light make only a confused impression on the retina. About a month after birth, the eye seems to have acquired that tension and solidity which are necessary for the proper transmission of the rays of light; but, even then, infants are incapable of fixing their eyes upon any [2] object: They roll and move them to all sides, without being able to distinguish the objects to which their eyes are directed. In six or seven weeks, however, they begin to fix their attention upon luminous objects. But this exercise only tends to fortify the eye, without conveying any exact perception of different objects; for the first great error in vision, is the inverted representation of objects upon the retina: And, till children learn the real position of bodies by the sense of feeling, they see every object inverted. A second error in the vision of infants arises from the double appearance of objects; because a distinct image of the same object is formed on the retina of each eye. It is only by the experience of feeling bodies, that children are enabled to correct this error; by the frequent handling of objects, they gradually learn that they are neither double nor inverted; and custom soon makes them imagine they see objects in the order and position in which they are represented to the mind by the sense of touching. Hence, if we were deprived of feeling, our eyes would deceive us, both with regard to the position and number of objects.
The inversion of objects is a result of the structure of the eye; for the rays which form the images of these objects, cannot enter the pupil without crossing each other. This admits of an easy proof: When light is transmitted through [3] a small hole into a dark chamber, the images of the objects from without are represented on the wall in an inverted position; because all the rays reflected from the different points of the object cannot pass through this small hole, in the same extent and position as they proceed from the object, unless the hole be of equal dimensions with the object. But, as every part of the object reflects images of itself on all sides, and, as the rays which form these images proceed from every point of the object as from so many centres, none of them can pass through the hole but those that arrive at it in different directions. Hence the hole becomes the centre of the whole object, at which the rays flowing from the lower, as well as the higher parts of the object, arrive in converging directions; and, of course, they must cross each other at this centre, and represent the picture of the object on the opposite wall in an inverted position.
It is equally easy to show that we see all objects double. If, for instance, we look at an object with the right eye, we will find that it corresponds with a certain point of the wall; if we look at the same object with the left, it then corresponds with a different point; and, lastly, when we look at it with body eyes, it appears in the middle between these points. Thus an image of the object is formed on both eyes, one of which appears on the left and the other on [4] the right; and we perceive it to be single and in a middle situation, because we have learned to correct this error of vision by the sense of touching. In the same manner, if we look with both eyes at two objects, nearly in the same direction, by fixing our eyes on the nearest, we perceive it to be single; but the farthest appears to be double; and, if we fix our eyes on the farthest, it appears to be single, while the nearest is perceived to be double. This is an evident proof, that we see all objects double, though we conceive them to be single; and that, though we form an accurate idea of their real situation, yet we actually see them where they are not. If, therefore, the sense of seeing were not constantly rectified by that of touching, we would be perpetually deceived as to the position, number, and situation of objects; we would perceive them to be inverted, double, and to the right or left of their real situations; and, instead of two, if we had 100 eyes, we would still conceive objects to be single, though they were in reality multiplied a hundred fold.
Thus a separate image of every object is formed in each eye; and, when the two images fall on corresponding parts of the retina, or those parts which are always affected at the same time, objects appear single, because we are accustomed to judge them in this manner. But, when the images of objects fall upon parts of the retina which are not usually affected at the same [5] time, they then appear double, because we have not acquired the habit of rectifying this unusual sensation.
Mr Chesselden, in his anatomy,* relates the case of a man who had been affected with a strabismus, in consequence of a blow on the head. This man saw every object double for a long time. But he gradually learned to correct this error of vision, with regard to objects which were most familiar to him; and, at last, he saw every object single as formerly, though the squinting of his eyes was never removed. This is a proof still more direct, that we really see all objects double, and that it is by habit alone we learn to conceive them to be single. If it should be asked, why children sooner acquire the faculty of correcting this deception than adults whose eyes have been distorted by accident? it may be replied, that children, having acquired no opposite habits, less time is, of course, necessary to correct the errors of their sensations; but that adults, who have for many years been accustomed to perceive objects single, because their images fall upon corresponding parts of the retina, have a contrary habit to oppose, and, consequently, must require a long time before they can efface all the traces of it.
The sense of seeing conveys no idea of distances. Without the aid of touching, all objects would appear to be within the eye, because it is [6] there alone that their images exist: And an infant, who has had no experience of the sense of touching, must consider all external bodies as existing in itself: They only appear larger or smaller, according as they approach or recede from the eye. A fly, when near the eye, will seem larger than an ox or a horse at a distance. Thus an infant can have no idea of the relative magnitude of objects, because he has no notion of the different distances at which he views them. It is only after measuring space by the extension of the hand, or by transporting their bodies from one place to another, that children acquire ideas concerning the distances and magnitudes of objects. Before this period, they can form no judgment of the distance or magnitude of an object, but by the image painted on the retina. Their ideas of magnitude entirely result from the angle formed by the extreme rays reflected from the superior and inferior part of the object: Of course, every near object must appear to be large, and every distant object small. But, after having acquired, by touch, ideas of distances, the judgment concerning magnitude begins to be rectified: They trust not alone to the apparent magnitude conveyed by the eye: They endeavour to investigate the distance; they try, at the same time, to distinguish the object by its form; and then they judge of its magnitude.
If we judge by the eye alone, and have not acquired the habit of apprehending the same ob- [7] jects to be equally large, though viewed at different distances, the first soldiers, in a file of 20, must appear much larger than the last. But we know the last soldier to be equally large with the first; and hence we judge him to be of the same dimensions. And, as we have the habit of considering the same object to be of equal magnitude at all ordinary distances, we are never deceived on this head, excepting when the distance is too great, or when the interval is an uncommon direction. A distance ceases to be familiar to us whenever it is too large, or rather when the interval is vertical instead of horizontal. The first ideas of the comparative magnitude of objects we acquire either by measuring their relative distances by the hand, or by moving the whole body. But all the experiments by which we commonly rectify the errors of vision, with regard to distances, are made horizontally. We have no acquired habit of judging of the magnitude of objects which are elevated above, or sunk below us; because we are not accustomed to measure in this direction by the touch. Hence, when viewing men from the top of a tower, or when looking up to a cock or a glove on the top of a steeple, we think these objects are much more diminished, than if we viewed them at equal distances in a horizontal direction.
Though a small degree of reflection be sufficient to convince us of the truth of these positions, it may still be of use to relate the facts [8] which confirm them. The celebrated Chesselden couched cataracts in both eyes of a lad of 13 years of age, who had been blind from his birth. The operation succeeded; and Mr Chesselden carefully observed the manner in which the young man was affected by the sense of seeing. These observations he published in the Philosophical Transactions.* This young man was not absolutely blind: Like other persons affected with cataracts, he could distinguish night from day, and even black from white; but he had not the most distant conception of the figure of bodies. The operation was first performed on one eye. When he saw for the first time, he was so far from judging distances, that he believed every object touched his eyes, in the same manner as every thing he handled touched his skin. Objects of a regular figure, and having plain surfaces, were most agreeable to him, though he was still incapable of forming any judgment as to their form, or telling why they afforded him more pleasure than others. His ideas of colours, before the operation, were so faint, that, after receiving his sight, he was unable to distinguish one from another. He insisted that the colours which he saw were not the same he was formerly acquainted with. He knew not the figure of any object; nor could he distinguish one from another, however different in form and in magnitude. [9] When presented with things which were formerly familiar to him, he observed them with attention, that he might be able to know them afterwards. But, as he had too many objects to recognise at once, he forgot the greatest part of them; and, from his commencing to distinguish objects, he did not retain in his memory one out of a thousand. Those objects and persons which were formerly most beloved by him, he was astonished to find that they were not also the most agreeable to his sight. It was more than two months before he could perceive that pictures were the representations of solid bodies. Previous to this period, he considered them only as plain surfaces diversified by different colours. But, after he began to perceive that pictures represented solid bodies, he expected to recognise their seeming inequalities by touching the canvas; and was perfectly astonished when he found the whole uniformly smooth. He asked, whether the deception arose from the sense of feeling or that of seeing? He was then shown a miniature portrait of his father, contained in his mother’s watch-case. He recognised the resemblance of his father: But he inquired with amazement how so large a countenance could possibly be contained in so small a compass; for it appeared to him equally strange, as that a bushel should be held in a pint vessel. At first, his eye could support only a small quantity of light; and every object seemed much larger [10] than the life. But, after he had seen objects of large dimensions, former objects appeared to be proportionally diminished. He had no conception that any object exceeded the limits of those he had already seen. He knew that his own apartment was only a part of the house, and yet he was unable to comprehend how the house should be larger than his chamber. Before the operation, he expected not much pleasure from the acquisition of the new sense that had been promised him, excepting what should arise from his being enabled to read and write. He alledged, for example, that he could receive no new satisfaction from walking in the garden, because he already knew every corner of it, and could walk there with great ease and freedom. He had even remarked, that his blindness gave him the advantage of walking in the night with more confidence and security than those who enjoyed the benefit of sight. But, after he began to have the proper use of this new sense, he was transported beyond measure. He declared that every new object afforded a fresh delight; and that the pleasure he felt exceeded the powers of expression. About twelve months after the operation, he was conducted to Epsom, from which there is a beautiful and extensive prospect. He was charmed with the view; and he called this landscape a new mode of seeing. [11]
About a year after the first operation, the cataract on the other eye was couched with equal success. With this second eye he perceived objects to be much larger than with the other, but not so large as when he first received sight; and, when he viewed the same object with body eyes, he said that it appeared to be twice as large as with the first eye alone. But, after he procured the use of both eyes, he did not see objects double, or, at least, Mr Chesselden could not be certain that he did.
Mr Chesselden records several other examples of blind men, who had no remembrance of light, restored to vision by the same operation; and he assures us, that, when they first obtained the use of their eyes, they expressed their perceptions in a similar manner, though not so minutely: And he remarks, upon the whole, that as, during their blindness, they had no occasion to move their eyes, it cost them much difficulty and a considerable time, before they could acquire the faculty of directing them to the objects they wished to examine.*
As, from particular circumstances, we can have no just idea of distance, and, as we cannot judge concerning the magnitude of objects, but by the largeness of the angle or image formed in the eye, we must necessarily be liable to deceptions with regard to these articles. Every body knows how liable we are, when travelling in the night, to mistake a bush that is near us [12] for a tree at a distance, or a distant tree for a bush which is at hand. In the same manner, if we are unacquainted with the figure of objects, we cannot form any idea, either of their distance or magnitude: A fly passing with rapidity at some inches from the eye, would, in this case, appear like a bird at a considerable distance; and a horse, standing in the middle of a plain, would not seem larger than a sheep. But, as soon as we knew it to be a horse, it would instantly appear as large as the life, because we have the power of correcting the deception of vision.
Whenever, therefore, we are benighted in a part of the country with which we are unacquainted, being unable, on account of the darkness, to judge of the distance or figure of objects, we are every moment liable to all the deceptions of vision. This is the source of that dread which most feel in the dark, and of those spectres and terrible figures which so many persons tell us they have seen in the night. Though such figures, it is commonly asserted, exist only in the imagination; yet they may have a real existence in the eye; for, whenever we have no other mode of judging of an unknown object but by the angle it forms in the eye, its magnitude will uniformly increase in proportion to its propinquity. If it appears, when at the distance of 20 or 30 paces, to be only a few feet high, its height, when within two or three [13] feet of the eye, will be many fathoms. An object of this kind must naturally excite terror and astonishment in the spectator, till he approaches and recognises it by actual feeling; for the moment a man knows an object, the gigantic appearance it assumed in the eye instantly diminishes, and its apparent magnitude is reduced to its real dimensions. But if, instead of approaching such an object, the spectator flies from it, he can have no other idea of it, but from the image which it formed in his eye; and, in this case, he may affirm with truth, that he saw an object terrible in its aspect, and enormous in its size. Thus the notions concerning spectres is founded in nature, and depend not, as some philosophers affirm, upon the imagination alone.
When we are unable to form an idea of the distance of objects by a knowledge of the space between them and the eye, we endeavour to judge of their magnitude by distinguishing their figures. But, when the figures are not distinguishable, and when we view a number of objects of the same form, we conceive those that are most brilliant to be nearest, and those which are most obscure to be at the greatest distance. This mode of judging gives rise to deceptions of a singular nature. When a multitude of objects are disposed in a right line, as the lamps on the road from Versailles to Paris, of the proximity or remoteness of which we can only judge by the different qualities of light they transmit to the [14] eye, it frequently happens, when viewed at the distance of an eighth of a league, that the lamps appear to be on the right hand, in place of the left. This description is an effect of the cause above mentioned; for, as the spectator has no other criterion to judge of the distance of the lamps, but the quantity of light they emit, he thinks the most brilliant of them is nearest to his eye. Now, if the first two or three lamps should happen to be most obscure, or, if one in the whole range was more brillant [sic] than the rest, that one, to a spectator, would seem to be the first, and all the others, whatever might be their real situation, would seem to be placed behind it. This apparent transposition could not be effected by any other means than a change of situation from left to right; for, in a long range of objects, we cannot apprehend what is really behind to be situated before any one of these objects, without seeing on the right what is on the left, or on the left what is on the right.
I have thus mentioned the principal defects of the sense of seeing; and shall now proceed to examine the nature, properties, and extent of that admirable organ by which we are enabled to have a communication with the most distant objects. Sight is a species of touching, but very different from the common species of that sense. Before we can touch any object, we must either approach it with some part of our body, or it must approach us. But, with the eye, we can [15] touch any object, however distant, if it transmits a sufficient quantity of light to make an impression on, or if its picture forms a sensible angle in, the eye. The smallest visible angle is a bout one minute. This angle, when an object is viewed as the greatest distance of vision, is about the 346th part of the diameter of that object. An object, for example, of a foot square, ceases to be visible at the distance of 3436 feet. A man of five feet high is not visible beyond the distance of 17,180 feet, when the sun shines.
But, with regard to the extent of human vision, an observation occurs, which seems to have escaped all the writers on optics: The extent of our sight diminishes or augments in proportion to the quantity of light that surrounds us, supposing the illumination of the object to remain the same. If the same object which we see during the day at the distance of 3436 times its diameter, were equally illuminated during the night, it would be visible at a distance 100 times greater. A candle is visible in the night at the distance of more than two leagues; that is, supposing the diameter of the luminary to be one inch, it would be visible at the distance of 316800 times the length of its diameter. But, in the day, this candle would not be discernible beyond ten or twelve thousand times the length of its diameter. The same remark is applicable to all objects, when viewed during the day or the [16] night. We may, therefore, conclude, that the extent of our vision is much greater than our first supposition; and that the reason why we are often unable to distinguish distant objects is less owing to a defect of light, or to the smallness of the angle under which they are painted in the eye, than to the profusion of rays reflected from intermediate objects, which, by their brilliancy, prevent us from perceiving the fainter and more diverging rays that proceed from distant objects. The retina of the eye is like a canvas upon which objects are painted. The colour of those pictures are bright or obscure, in proportion to the distances of the objects represented. When objects are very remote, their pictures on the retina are so faint, that they are entirely obliterated by the vigorous and lively impressions made on the eye by nearer objects, with which we are every when environed. But, when the intermediate objects emit only a feeble light, compared with that which proceeds from remote objects, as, for example, when we view a luminous body in the night-time, then the distant object makes a distinct picture on the retina, and become perfectly visible. It is a consequence of these facts, that a man, by placing himself in the dark, and employing a long tube, may make a telescope, which will have a considerable effect even during the day. For the same reason, a man at the bottom of a deep pit can see the stars at noon; and this fact was not [17] unknown to the antients, as appears from the following passage of Aristotle: “Manu enim admota, aut per sistulam, longius cernet. Quidam ex foeveis puteisque interdum stellas conspiciunt.”
We may, therefore, affirm, that the human eye is capable of being affected with objects which subtend not an angle above a second, or less, even when they reflect no more light than when they were seen under an angle of one minute; and, consequently, that the powers of this organ are greater than was formerly imagined. But, if objects, without forming a greater angle, were furnished with a more intense light, we would see them at still greater distances. A small taper, when vivid, is seen much farther than a flambeau that emits a dim light. In order to determine the utmost distance at which an object can be rendered visible, three things fall to be considered: 1. The largeness of the angle formed in the eye; 2. The degree of light with which the neighbouring and intermediate objects are illuminated; and, 3. The intensity of the light proceeding from the object itself. Vision is affected by each of these causes; and it is only by estimating and comparing them, that we can determine the distance at which any particular object can be discerned. The following is a demonstrative proof of the influence of the intensity of light upon vision. Telescopes and microscopes are known to be instruments of the [18] same kind, each of them increasing the visible angle of objects, whether they be really minute, or appear so on account of their distance. Why then do telescopes with difficulty magnify objects a thousand times, when a good microscope magnifies them more than a million? This difference, it is apparent, proceeds only from the degree of light; for, if we could illuminate distant objects with an additional quantity of rays, they would appear infinitely clearer, though seen under the same angle; and telescopes would have the same effect upon distant objects as microscopes have upon minute bodies. But this is not a proper place for expatiating on these subjects.
The distance at which any object can be seen is seldom the same in both eyes. There are few men who have both eyes equally strong. When this inequality is great, the strongest eye is most generally employed, which is the cause of squinting, as I have elsewhere proved.* When both eyes are equally strong, and directed to the same object, one would imagine that the vision would be doubly distinct; but the difference has ben found by experiment to be only one 13th part,+ and this phaenomenon may admit of the following solution. The two optic nerves, near the place where they come out of the skull, unite and then separate by an obtuse angle before they [19] enter the eyes. The motion communicated to these nerves by the impression of objects on the retina, cannot be transmitted to the brain without passing the united part. Hence these two motions must be combined, and produce a similar effect, as when two bodies moving upon two sides of a square, and impinging on a third, make it move in the diagonal. Now, if the angle were about 115 or 116 degrees, the diagonal would be to the side as 13 to 12, which is the same ratio that the sensation resulting from both eyes bears to that which results from one. The angle formed by the two optic nerves being nearly equal to that above supposed, the loss of sensation may be attributed to this position of the nerves; and this loss will always increase in proportion to the greatness of the angle.
Short-sighted persons are generally supposed to see objects larger than other men: But the reverse is the truth; for they actually see them diminished. I myself am short-sighted, and my left eye is stronger than my right. I have a thousand times examined the same objects, as the letters of a book, at the same distance, first with the one eye, and then with the other, and uniformly found that objects appeared both clearest and largest to the left eye; and, when I distorted one of my eyes to make an object appear double, the image presented to the right eye was less than the other. I cannot, therefore, hesitate in pronouncing, that the more short-sighted any [20] man is, he sees objects proportionally diminished. I examined several persons who had eyes unequal in strength, and all of them declared that they saw objects larger with the strong than with the weak eye. This phaenomenon is perhaps the effect of habit; for short-sighted people, being accustomed to approach close to objects, and to view only a small portion of them at a time, their eyes acquire a standard of magnitude much less than other men, who can take in at once all the parts of larger bodies.
Short-sightedness has been often ascribed to a roundness of prominence of the eyes. But this cause is not satisfactory; for some have suddenly become short-sighted, as the young man mentioned by Mr Smith in his optics,* who became short-sighted on coming out of a cold bath, and who, from that period, was always obliged to use a concave glass. It cannot be supposed that the crystalline and vitreous humours were all at once inflated to such a degree as to produce this difference in vision. Short-sightedness may as well proceed from the respective position of the different parts of the eye, and especially of the retina, as from the form of the humours; it may proceed from a less degree of sensibility in the retina, from a smallness of the pupil, &c. In the two latter cases, it is true, concave glasses would be useless, and even hurtful; in the two former, they may be employed with advantage. [21] But still, objects seen through these glasses are neither so distinct, nor perceived at such a distance, as other men see them with the naked eye; because short-sighted persons, as formerly remarked, see the pictures in a diminished form, and concave glasses diminish them still farther: Whenever, therefore, these pictures become so small as to make too faint an impression on the retina, they cease to be visible; consequently, people who labour under this defect, see not so far with the assistance of glasses as other men do with their eyes.
As the eyes of infants are less than those of adults, they must likewise see objects less; because the greatest angle which an object can form in the eye must always be proportioned to the dimensions of the retina: If the field of the retina, where the pictures of objects are formed, be supposed to be half an inch in adults, it will not exceed a third or a fourth of an inch in infants. Children, of course, cannot see so far as adults; for, as objects appear less to them, they must sooner become invisible. But as, infants, the pupils are larger, in proportion to the size of their eyes, than those of adults, they m ay derive some small advantage from this circumstance.
Old men, as the humours of their eyes are said to be dried up, ought to see nearer than young men: But the reverse is true; for old [22] men see best at a distance. This alteration cannot proceed entirely from a diminution, or a flattening of the humours of the eye, but rather from a change of position between its parts, as between the cornea and crystalline, or between the vitreous humour and the retina. This may be easily understood, by supposing that the cornea becomes more solid as we advance in years, and, consequently, that it cannot readily assume that convexity which is necessary in order to see near objects; and, as it must be flattened by drying, this circumstance alone is sufficient to make old men see best at a distance.
Clear and distinct vision, though different in their nature, are terms very generally confounded by writers on optics. We see an object clearly, whenever it is sufficiently illuminated to enable us to form a general idea of its figure; but we see it not distinctly till it be so near that we can recognize all its parts. When we view a distant tower, we see it clearly as soon as we perceive it to be a tower; but we see it not distinctly, till we approach so near as to be able to determine not only its general dimensions, but to distinguish the parts of which it is composed, as the order of architecture, the materials, the windows, &c. We may, therefore, see an object clearly without seeing it distinctly, and we may see it distinctly without seeing it clearly; because [23] distinct vision implies a successive examination of the different parts of objects. Old men see clearly, but not distinctly: They perceive large or luminous objects at a distance; but they are unable to distinguish small objects, as the characters of a book, without the assistance of magnifying glasses. Short-sighted persons, on the contrary, see small objects distinctly; but they have no clear vision of large objects, unless they are diminished by concave glasses. A great quantity of light is necessary for clear vision, and a small quantity is sufficient for distinct vision. Hence short-sighted people see better in the night than other men.
When an object is too brilliant, or when the eyes fixes too long upon the same object, the organ is injured and fatigued, vision becomes indistinct, and the image of the object, having made an impression too violent, or remained too long on the retina, seems, for some time, to be pained on every body we look at. But I will not enlarge on this subject, because I have elsewhere given a full explication of it.* I shall only observe, that nothing, perhaps, is more destructive to the eye than too great a quantity of light. Blindness is exceedingly frequent in the northern regions, where the snow, illuminated by the rays of the sun, obliges travellers to cover their eyes with crape, to prevent the [24] dangerous, and often sudden, effects of too much light. In the sandy desarts [sic] of Arabia, the reflection of the light is so violent, that they eyes are unable to support it. Such persons, therefore, as are obliged to write or read long at a time, should beware of using a strong light.
* P. 324 [back to page 6].
* See Phil. Trans. No. 402. and Tatler, Art. 55 [back to page 9].
* See Mem. De l’Acad. annee 1743 [back to page 19].
+ See Jurin’s essay on distinct and indistinct vision [back to page 19].
* Vol. 2. p. 10 [back to page 21].
* See Mem. De l’Acad. annee 1743 [back to page 24].