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 when we 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 their real 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 of the 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 farther off 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 have examined, 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 and distinct 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; and travellers, 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.