YEARS OF LIFE.
YEARS OF LIFE.
YEARS OF LIFE.
YEARS OF LIFE.
YEARS OF LIFE.
YEARS OF LIFE.
From these tables many useful conclusions might be drawn. But I shall only consider those which respect the probabilities of the duration of life. It is observable, that in the columns opposite the years 10, 20, 30, 40, 50, 60, 70, and other round numbers as 25, 35, &c. the deaths in the country parishes are more numerous than in in the preceding or subsequent columns. The cause of this seeming inconsistency arises from the generality of country people being ignorant of their exact age, and therefore if they die at 58 or 59 in the parish register it is entered 60; and so of other round numbers. From this irregularity the inconvenience is not great, as it may easily be corrected by the manner in which the numbers succeed each other in the Tables.
By the tables in the country parishes it appears, that almost one half of the children die before the age of four years, and by the Paris table not before 16; which great difference; certainly arises from the children being sent into the country to nurse, and consequently increases the number of deaths there in infancy. As likely to come at the truth, I have blended the two tables, and from thence calculated the probabilities of the duration of life as follows:
TABLEof thePROBABILITIESof theDURATIONofHUMAN LIFE.
By this table it appears, that an infant newly born has an equal chance of living eight years; that an infant of one will live 33 years longer; that a child of two will live 38 years longer; that a man of 20 will live 33 years and five months longer; that a man of 30 will live 28 years longer; and so proportionally of every other age.
It is also to be observed, first, that seven years is the age at which the longest duration of life is to be expected, since there is then an equal chance of living 42 years and three months longer; secondly that at the age of 12 one fourth of our existence is gone, as we cannot in reason expect above 38 or 39 years more; thirdly, that we have enjoyed one half of our existence at the age of 28, as we can reckon upon only 28 years more; and lastly, that by the age of 50 three fourths of life are passed, the remaining probability being only for 16 or 17 years.
But these physical truths, however mortifying, may be compensated by moral considerations. A man ought to consider as nothing the first fifteen years of his life. Every thing that happens in that long interval of time is effaced from the memory, or has at least solittle connection with the views and objects Which afterwards occupy our thoughts that it gives us no concern. Neither, indeed, have we the game succession of ideas, nor, it maybe said, the same existence. In a moral sense we do not begin to live till we have begun to regulate our thoughts, to direct them towards futurity, and to assume to ourselves a kind of consistency of character conformable to that state which has some relation to what we shall afterwards become. By considering the duration of life in this, the only real point of view, we shall find, that at the age of 25 we have passed but one fourth part of our life; at the age of 38 one half; and that, at the age of 56, there is one fourth of life still remains.
CHAPTER VI.
OF THE SENSE OF SEEING.
Having described the parts of which the human body consists, let us now proceed to examine its principal organs; the expansion of the senses, and their several functions; andat the same time, point out the errors to which, through them, we are in some measure subjected by Nature.
The eyes seem to be formed very early in the human embryo. In the chicken also, of all the double organs they are the soonest produced. I have observed in the eggs of several sorts of birds, and in those of lizards, that the eyes were more large, and early in their expansion, than any other parts of a two-fold growth. Though, in viviparous animals, and particularly in man, they are, at first, by no means so large in proportion as in the oviparous, yet they obtain their due formation sooner than any other parts of the body. It is the same with the organ of hearing; the small bones of the ear are entirely formed before any of the other bones have acquired any part of their growth or solidity. Hence it is evident, that the parts of the body, which are furnished with the greatest quantity of nerves, as the ears and eyes, are those which first appear, and which are the soonest brought to perfection.
If we examine the eyes of an infant, a few hours after its birth, we shall discern that it cannot make the smallest use of them; the organ not having acquired a sufficient consistency,the rays of light strike but confusedly upon the retina. Before the sixth week, children turn their eyes indiscriminately upon every object, without appearing to be affected by any, but at about this time they begin to fix them upon the most brilliant colours, and seem peculiarly desirous of turning them towards the light; this exercise does not give any exact notion of objects, but strengthens the eye, and qualifies it for future vision.
The first great error in the sense of seeing, is the inverted representation of objects upon the retina, and, till the sense of feeling has served to undeceive it, the child actually beholds every thing upside down. The second error in early vision is, that every object appears double; from the same object being formed distinctly upon each eye. This illusion like the other, can only be corrected by children from their being in the practice of handling different objects, and from which practice alone it is that they learn things are neither inverted nor double, and custom induces them to believe they see objects in the position the touch represents them to the mind; and therefore, were we denied the sense of feeling, thatof seeing would not only deceive us as to the situation, but as to the number of every object around us.
We may easily be convinced that objects appear inverted, (which arises from the structure of the eye) by admitting the light to pass into a darkened room through a small aperture, when the images of the objects without will be represented upon the wall in an inverted position; for, as all the rays which issue from their different points cannot enter the hole in the same extent and position which they had in leaving the object, unless the aperture was as large as the object itself; as every part of the object sends forth its image on all sides; as the rays which from those images flow from all points of the object, as from so many centres, those only can pass through the small aperture which come in opposite directions. Thus the hole becomes a centre for the entire object, through which the rays from the upper, as well as from the lower parts of it, pass in converging directions; and, of consequence, they must cross each other at this centre, and thus represent the objects upon the wall, in an inverted position.
That we, in reality, see all objects double, is also evident; for example, if we hold up a finger, and look with the right eye at an object, it will appear against one particular part of the room, shutting that and looking with the left, it will seem to be on a different part, and if we open both eyes, the object will appear to be placed between the two extremes. But the truth is, the image of the object is formed in both eyes, one of which appears to the right, and the other to the left; and it is from the habit of touching that we suppose we see but one image placed between them both. From which it is clear that we see all objects double, although our imagination forms them single; and that, in fact, we see things where they are not, notwithstanding we have a pretty exact idea of their situation and position; and thus it is that till the sense of feeling has corrected the errors of sight, if instead of two eyes, we had an hundred, we should still fancy the objects single, although they were multiplied an hundred times.
In each eye, therefore, is formed a separate image of every object; and when the two images strike the correspondent parts of the retina, that is, the parts which are alwaysaffected at the same time, the object appears single: but, when the images strike the parts of the retina, which are not usually affected together, then it appears double, because we are not habitual to this unusual sensation, and are then somewhat in the situation of infants just beginning to exert the faculty of vision.
M. Chesselden relates the case of a man, who, in consequence of a blow on the head, became squint-eyed, and saw objects double for a long time: but who was at length enabled, by slow and gradual steps, to see them singly as he had formerly done, notwithstanding the squinting remained. Is not this a proof, still more evident, that in reality we see things double, and that it is by habit alone we conceive them to be single? Should it be asked why children require less time, in order to see things single, than persons more advanced in years, whose eyes may have been affected by accident? it might be answered, that the sensations of children, being unopposed by any contradictory habit, these errors are rectified with ease; but that persons who have for many years seen objects single, because they affected the two correspondent parts of the retina, and who now see them double, labour under the disadvantageof having a contrary habit to oppose, and must therefore be a considerable time before it is entirely obliterated.
By the sense of seeing we can form no idea of distances; without aided by the touch, every object would appear to be within our eyes; and an infant, that is as yet a stranger to the sense of feeling, must conceive that every thing it sees exists within itself. The objects only appear to be more or less bulky as they approach to, or recede from the eye; insomuch, that a a fly near the eye will appear larger than an ox at a distance. It is experience alone that can rectify this mistake; and it is by constantly measuring with the hand, and removing from one place to another, that children obtain ideas of distance and magnitude. They have no conception of size but from the extreme rays reflected from the object, of course every thing near appears large, and those at a distance small. The last man in a file of soldiers appears much more diminutive than the one who is nearest to us. We do not, however, perceive this difference, but continue to think him of equal stature; for the number of objects we have seen thus lessened by distance, and found by repeated experience to be of the natural sizewhen we come closer, instantly correct the sense, and therefore we perceive every object nearly in its natural proportion, unless when we observe them in such situations as have not allowed us sufficient experience to correct the illusions of the eye. If, for example, we view men upon the ground, from a lofty tower, or look up to any object upon the top of a steeple, as we have not been in the habit of correcting the sense in that position, they appear to us exceedingly diminished, much more so than if we saw them at the same distance in an horizontal direction.
Though a small degree of reflection may serve to convince us of the truth of these positions, yet it may not be amiss to corroborate them by facts which cannot be disputed. M. Chesselden, having couched for a cataract a lad of thirteen years of age, who had from his birth been blind, and thus communicated to him the sense of seeing, was at great pains to mark the progress of his visual powers; his observations were afterwards published in the Philosophical Transactions. This youth was not absolutely and entirely blind: Like every other person, whose vision is obstructed by a cataract, he could distinguish day from night and evenblack from white, but of the figure of bodies he had no idea. At first the operation was performed only upon one of his eyes, and when he saw for the first time, so far was he from judging of distances, that he supposed (as he himself expressed it) every thing he saw touched his eyes, in the same manner as every thing he felt touched his skin. The objects that pleased him most were those whose surfaces were plain and the figures regular, though he could in no degree judge of their different forms, or assign why some were more agreeable to him than others. His ideas of colours during his former dark state were so imperfect, that when he saw them in reality he could hardly be persuaded they were the same. When such objects were shewn him as he had been formerly familiar with by the touch, he beheld them with earnestness, in order to know them again, but as he had too many to retain at once the greatest number were forgotten, and for one thing which he knew, after seeing it, there were a thousand, according to his own declaration, of which he no longer possessed the smallest remembrance. He was very much surprised to find that those persons, and those things, which he had loved best, were not themost pleasing to his sight; nor could he help testifying his disappointment in finding his parents less handsome than he had conceived them to be. Before he could distinguish that pictures resembled solid bodies above two months elapsed; till then he only considered them as surfaces diversified by a variety of colours; but when he began to perceive that these shadings actually represented human beings, he expected also to find their inequalities; and great was his surprise to find smooth and even what he had supposed a very unequal surface; and he inquired whether the deception existed in feeling or seeing. He was then shewn a miniature portrait of his father, which was contained in his mother’s watch-case, and though he readily perceived the resemblance, yet he expressed his amazement how so large a face could be comprised in so small a compass; to him it appeared as strange as that a pint vessel should contain a bushel. At first he could bear but a very small quantity of light, and every object appeared larger than the life; but in proportion as he observed objects that were in reality large, he conceived the others to be equally diminished. Beyond the limits of what he saw he had no conceptionof any thing. He knew that the apartment he occupied was only a part of the house, and yet he could not imagine how the latter should be larger than the former. Before the operation he formed no great expectations of the pleasure he should receive from the new sense he was promised, excepting that thereby he might be enabled to read and write. He said among other things, that he could enjoy no greater delight from walking in the garden, because there he already walked at his ease, and was acquainted with every part of it. He also remarked that his blindness gave him one advantage over the rest of mankind, namely, that of being able to walk in the night with more confidence and security. No sooner, however, had he began to enjoy his new sense than he was transported beyond measure; he declared that every new object was a new source of delight, and that his pleasure was so great he had not language to express it. About a year after, he was carried to Epsom, where there is a beautiful and extensive prospect; with this he seemed greatly charmed; and the landscape before him he called a new method of seeing. He was couched in the other eye a year after the former, and the success was equally great.Every object appeared larger when he looked at it with the second eye to what it did with the other; and when he looked at any thing with both eyes it appeared twice as large as when he saw but with one, though he did not see double, or at least he shewed no marks from which any such conclusion might be drawn.
Mr. Chesselden instances several other persons who were in the same situation with this lad, and on whom he performed the same operation; and he assures us, that on first obtaining the use of their eyes they expressed their perceptions in the same manner, though less minutely; and that he particularly observed of them all, that as they had never had any occasion to move their eyes while deprived of sight, they were exceedingly embarrassed in learning how to direct them to the objects they wished to observe.
As from particular circumstances we cannot from a just idea of distance, and as we cannot judge of the magnitude of objects but by the largeness of the angle, or rather the image, which they form in our eyes, we are necessarily deceived as to the size of such objects. Every man knows how liable we are,in travelling by night, to mistake a bush which is at hand for a tree at a distance, or a tree at a distance for a bush which is at hand. In like manner, if we cannot distinguish objects by their figure we cannot judge of distance or size. In this case a fly, passing with rapidity before our eyes, will appear to be a bird at a considerable distance; and an horse standing in the middle of a plain will appear no bigger than a sheep till we have discovered that it is a horse, and then we shall recognize it to be as large as life.
Whenever, therefore, we find ourselves benighted in an unknown place, where upon account of the darkness no judgment is to be formed of distance, or figures of the objects that may present themselves, we are every moment in danger of being misled with respect to our ideas of such objects. Hence proceeds that internal fear and dread which most men experience from the obscurity of night, and of those strange and hideous spectres and gigantic figures which so many persons tell us they have seen. Though such figures, it is commonly asserted, exist solely in the imagination, yet they may appear literally to the eye, and be in every respect seen as described to us; for whenwe reflect that whenever we cannot judge of an unknown object but by the angle which it forms in the eye, this object is magnified in proportion to its propinquity; and that if it appears at the distance of twenty or thirty paces to be only a few feet high, when advanced within a short space of it, it will seem to be of considerable magnitude. At this the spectator must naturally be astonished and terrified, till he approaches and knows it by feeling; for in the very instant that he has an actual perception of what it is, the tremendous form it assumed to the eye will diminish, and it will appear in no other than its real and absolute form. If, on the other hand, he is afraid to approach it, and flies from the spot with precipitation, the only idea he can have of it will be that of the image which had been formed in his eye; the image of a figure he had seen, gigantic in its size, and horrible in its form. The prejudice with respect to spectres, therefore, originates from Nature, and depend not, as some philosophers have supposed, solely upon the imagination.
When we cannot form an idea of distance, by the knowledge of the intermediate space between us and any particular object, we endeavour to distinguish the form of that object,in order to judge of its size; but when we cannot perfectly distinguish the figures, and at the same time behold a number of objects, whose forms are correspondent, we conceive those which are most brilliant are most proximate, and those most obscure are most remote; a notion which is not unoften the source of very singular mistakes. In a multitude of objects disposed in a right line, as the lamps upon the road from Versailles to Paris, of which, as we cannot judge of the proximity or remoteness but by the quantity of light they transmit to the eye, it often happens that when examined at the distance of the eighth of a league, we see all the lamps situated on the right hand instead of the left, on which they are in reality situated. This fallacious appearance is produced from the above-mentioned cause, for as the spectator has no evidence of the distance he is from the lamps, but by the quantity of light they emit, so he conceives that the most brilliant lamps are those which are the first and the nearest to him. Now if some of the first lamps happen to be dull and obscure, and any one of the others particularly bright, that one would appear to be first and the rest behind, whatever was theirreal situation; and this seeming transposition would be solely owing to the supposed change of their situation from the left hand to the right; for to conceive to be before what is actually behind in a long file, is to see on the right what is situated on the left, or on the left what is situated on the right.
We may fairly consider sight as a species of touching, though very different from what we commonly understand by that sense; for in order to exercise the latter we must be near the object, whereas we can touch with the eye as far as the light the object contains will make an impression, or its figure form an angle therein. This angle, when the object is viewed at the greatest distance, is about the 3436th part of its diameter, therefore an object of a foot square is not visible beyond 3436 feet, or a man of five feet high at a greater distance than 17180 feet. But the extent of vision is in some measure influenced by the light which surrounds us, and we should be enabled to see any object in the night at 100 times greater distance than in the day, provided it was equally illuminated; thus, for instance, we can perceive a lighted candle at full two leagues in the night, supposing the diameter ofthe luminary to be one inch, whereas in the day we should not be able to discern it beyond the proportion of the above ratio; and as this is a circumstance which attends all objects when viewed at those different periods, we may conclude that one principal reason for our not being able to discern things at a greater distance, is the brilliancy of the light which fills up the intermediate space, and so destroys the reflected rays from those still more distant objects. When we are surrounded with strength of light the objects near make a forcible impression on the retina of the eye, and obliterate those far off, which are weak and faint; and, on the contrary, if we view a luminous body in the night, even at a considerable distance, that becomes perfectly visible, while those which are near are scarcely discernible. From these reasons it is, that a man at the bottom of a deep pit can see the stars, or, by employing a long tube in a dark room, may obtain some effects from the telescope in the middle of the day. From this it is evident, that if bodies were furnished with more strength of light they would be visible at greater distances, although the angle was not increased, for a small candle, which burns bright, is seen much fartheroff than a flambeau that is dim. Of these facts, relative to the influence of light, we have a still stronger proof in the variation between a microscope and telescope, both of them instruments of the same kind, increasing the visible angles of objects, whether they be really minute or rendered so by distance, and yet the latter does not magnify beyond a thousand times, whereas the former will exceed a million, and this difference plainly arises solely from the degree of light, for could the distant object be additionally illuminated, telescopes would have the same effect upon distant objects as microscopes have upon small bodies. But it is only by comparing the size of the angle formed in the retina of the eye, the degree of light which illuminates the adjacent and intermediate objects, and the strength of light which proceeds from, or is reflected by the object itself, that we can conclude upon the distance at which any particular body will be visible.
The power of seeing objects at a distance is very rarely equal in both eyes. When this inequality is great, the person so circumstanced generally shuts that eye with which he sees the least, and employs the other with all its power,and which is one cause of squinting. The object does not appear doubly distinct, by both eyes being placed upon it although they are equally strong, but has frequently been proved not to exceed a 13th part more than if beheld with one; and this is supposed to arise from the two optic nerves uniting near the place they came out of the skull and then separating by an obtuse angle before they enter the eyes; but as the motion made by the impression of objects cannot pass to the brain without passing this united part, the two motions must therefore be combined, and, consequently, cannot act with that force as though they were distinct; but from repeated experiments seem to bear the proportion above stated.
There are many reasons to suppose that short-sighted persons see objects larger than others; and it is a certain truth that they see them less. I am myself short-sighted, and my left-eye is stronger than my right. A thousand times have I experienced, upon looking at any object, as the letters of a book, that they appear least to the weakest eye; and that, when I place the book so that the letters appear double, the images of the left-eye, are greater than those of the right. Several others, I haveexamined, who were in similar circumstances, and I have always found, that the eye which saw every object best, saw it also largest. This may be ascribed to particular habits; for near-sighted people being accustomed to approach close to the object, and to view but a small part of it at a time, they acquire a small standard for magnitude, and when the whole of the object is seen, it necessarily appears smaller to them than to others, whose vision is more enlarged.
There have been many instances of persons becoming short-sighted on a sudden, therefore attributing it to the roundness or prominence of the eye is by no means certain. Mr. Smith, in his Optics, speaks of a young man that became short-sighted as he quitted a cold bath, and who was under the necessity of using a concave glass all his life after; and it cannot be supposed that the vitreous humours were instantly inflated so as to cause this difference in vision. Short-sightedness may arise from the position of the various parts of the eye, especially the retina, from a less degree of sensibility in the retina, or the smallness of the pupil. In the two first cases a concave glass may be used to advantage, but yet objects will not be seen so far, or so distinct,through these glasses as others will perceive with the eye alone, for as short-sighted persons see objects in a diminished form, the concave glass diminishes them still farther.
Infants having their eyes smaller than those of adults, must of consequence, see objects smaller also. For as the image formed on the back of the eye must be large, as the eye is capacious, so infants, having it not so great, cannot have so large a picture of the object. This may likewise be a reason, why they are unable to see so distinctly, or at such distances, as persons who have attained the years of maturity, for as objects appear less they must sooner become invisible.
Old people see bodies close to them very indistinctly, but bodies at a great distance from them with more precision, than young ones. This may happen from an alteration in the coats, or perhaps the humours of the eye; and not, as is supposed, entirely from their diminution. The cornea, for instance, may become too rigid to adapt itself, and take a proper convexity for seeing near objects, as a flatness must be occasioned by drying that will be sufficient of itself to render their eyes more calculated for distant vision. Although clear anddistinct are frequently confounded by writers on optics, yet they are very different; for we may be said, for instance, to clearly see a tower, as soon as we get a view of it, but we must approach near enough to distinguish its component parts before we see it distinctly. Men in years see clearly, but not distinctly; they can discern large bodies at a distance, but cannot distinguish small objects, as the characters in a book, without the help of magnifying glasses. On the contrary, short-sighted people see small objects distinctly, but need the aid of concave glasses to reduce large ones. Much light is also necessary for clear sight, while a small quantity is sufficient for distinct vision.
When an object is extremely brilliant, or we fix our eyes too long upon the same object, the organ is hurt and fatigued, vision becomes indistinct, and the image of the object, having made too violent an impression, appears painted on every thing we look at, and mixes with every object that occurs. How dangerous the looking upon bright and luminous objects is to the sight, is evident from the effect it has on the inhabitants of countries which are covered for the greatest part of the year with snow; andtravellers, who cross those countries, are obliged to cover their eyes with crape. In the sandy plains of Africa, the reflection of the light is so strong, that it is impossible for the eye to sustain the effects of it. Such persons therefore, as write, or read for any continuance, should chase a moderate light, for though it may seem insufficient at first, yet the eye will gradually become accustomed to the shade; and at any rate, it will be less injured by too little light than by too much.
CHAPTER VII.
OF THE SENSE OF HEARING.
As the sense of hearing, as well as that of seeing, gives us perceptions of remote objects, so it is subject to similar errors, and may deceive us, when we cannot rectify bythe touch, the ideas which it excites. It communicates no distinct intelligence of the distance from whence a sounding body is heard: a great noise far off, and a small one near, produce the same sensation, and, unless we receive aid from some other sense, we can never distinctly tell whether the sound be a great or a small one. It is not till we have, by experience, become acquainted with any particular sound that we can judge of the distance from whence we hear it; but if, for example, we hear the sound of a bell, we are at no great loss to determine its distance, any more than we are of that of a cannon from the report, judging in both cases from similar sounds, which we have been previously acquainted with.
Every body that strikes against another produces a sound which is simple in bodies non-elastic, but is often repeated in such as are elastic. If we strike a bell, a single blow produces a sound, which is repeated while the sonorous body continues to vibrate. These undulations succeed each other so fast, that the ear supposes them one continued sound; whereas, in reality, they form many. A circumstance of this kind happened to myself, for lying on the bedhalf asleep, I distinctly counted five strokes of the hammer upon the bell of the clock, and rising immediately found it was but the hour of one, and was convinced by examining the machinery that it had struck no more. A person, therefore, who should for the first time, hear the toll of a bell, would very probably be able to distinguish these breaks of sound; and, in fact, we can readily ourselves perceive remission in sounds.