Plate 6Page 28.Chap. 66.Plate 6.London, Published by J. Taylor High Holborn.
Page 28.Chap. 66.Plate 6.London, Published by J. Taylor High Holborn.
Page 28.
Chap. 66.
Plate 6.
London, Published by J. Taylor High Holborn.
Plate 7Page 28.Chap. 67.Plate 7.London, Published by J. Taylor High Holborn.
Page 28.Chap. 67.Plate 7.London, Published by J. Taylor High Holborn.
Page 28.
Chap. 67.
Plate 7.
London, Published by J. Taylor High Holborn.
Thelegs, or centre of support, in men and animals, will approach nearer to the centre of gravity, in proportion to the slowness of their motion; and, on the contrary, when the motion is quicker, they will be farther removed from that perpendicular line.
Thethickness or breadth of the parts in the human body will never be equal on each side, if the corresponding members do not move equally and alike.
Allbipeds in their motions lower the part immediately over the foot that is raised, more than over that resting on the ground, and the highest parts do just the contrary. This is observable in the hips and shoulders of a man when he walks; and also in birds in the head and rump.
Thehighest parts of quadrupeds are susceptible of more variation when they walk, than when they are still, in a greater or less degree, in proportion to their size. This proceeds from the oblique position of their legs when they touch the ground, whichraise the animal when they become straight and perpendicular upon the ground.
Themotion performed by a man, or any other animal whatever, in walking, will have more or less velocity as the centre of their weight is more or less removed from the centre of that foot upon which they are supported.
Thatfigure will appear the swiftest in its course which leans the most forwards.
Any body, moving of itself, will do it with more or less velocity in proportion as the centre of its gravity is more or less removed from the centre of its support. This is mentioned chiefly in regard to the motion of birds, which, without any clapping of their wings, or assistance of wind, move themselves. This happens when the centre of their gravity is out of the centre of their support, viz. out of its usual residence, the middle between the two wings. Because, if the middle of the wings be more backward than the centre of the whole weight, the bird will move forwards and downwards, in a greater or less degree as the centre of its weight is more or less removed from the middle of its wings. From which it follows, that if the centre of gravity be far removed from the other centre, the descent of the bird will be very oblique; but if that centre be near the middle of the wings, the descent will have very little obliquity.
Plate 8Page 31.Chap. 74.Plate 8.London, Published by J. Taylor High Holborn.
Page 31.Chap. 74.Plate 8.London, Published by J. Taylor High Holborn.
Page 31.
Chap. 74.
Plate 8.
London, Published by J. Taylor High Holborn.
A manmoving against the wind in any direction does not keep his centre of gravity duly disposed upon the centre of support[18].
Theman who rests upon his feet, either bears the weight of his body upon them equally, or unequally. If equally, it will be with some accidental weight, or simply with his own; if it be with an additional weight, the opposite extremities of his members will not be equally distant from the perpendicular of his feet. But if he simply carries his own weight, the opposite extremities will be equally distant from the perpendicular of his feet: and on this subject of gravity I shall write a separate book[19].
Thenavel is always in the central or middle line of the body, which passes through the pit of the stomach to that of the neck, and must have as much weight, either accidental or natural, on one side of the human figure as on the other. This is demonstrated by extending the arm, the wrist of which performsthe office of a weight at the end of a steelyard; and will require some weight to be thrown on the other side of the navel, to counterbalance that of the wrist. It is on that account that the heel is often raised.
Aftera man, by standing long, has tired the leg upon which he rests, he sends part of his weight upon the other leg. But this kind of posture is to be employed only for old age, infancy, or extreme lassitude, because it expresses weariness, or very little power in the limbs. For that reason, a young man, strong and healthy, will always rest upon one of his legs, and if he removes a little of his weight upon the other, it is only a necessary preparative to motion, without which it is impossible to move; as we have proved before, that motion proceeds from inequality[20].
Ifthe figure rests upon one foot, the shoulder on that side will always be lower than the other; and the pit of the neck will fall perpendicularly over the middle of that leg which supports the body. The same will happen in whatever other view we see that figure, when it has not the arm much extended, nor any weight on its back, in its hand, or on its shoulder, and when it does not, either behind or before, throw out that leg which does not support the body.
Plate 9Page 32.Chap. 78.Plate 9.London, Published by J. Taylor High Holborn.
Page 32.Chap. 78.Plate 9.London, Published by J. Taylor High Holborn.
Page 32.
Chap. 78.
Plate 9.
London, Published by J. Taylor High Holborn.
Plate 10Page 33.Chap. 80.Plate 10.London, Published by J. Taylor High Holborn.
Page 33.Chap. 80.Plate 10.London, Published by J. Taylor High Holborn.
Page 33.
Chap. 80.
Plate 10.
London, Published by J. Taylor High Holborn.
Theextending of the arm, which was bent, removes the weight of the figure upon the foot which bears the weight of the whole body: as is observable in rope-dancers, who dance upon the rope with their arms open, without any pole.
Theshoulder which bears the weight is always higher than the other. This is seen in the figure opposite, in which the centre line passes through the whole, with an equal weight on each side, to the leg on which it rests. If the weight were not equally divided on each side of this central line of gravity, the whole would fall to the ground. But Nature has provided, that as much of the natural weight of the man should be thrown on one side, as of accidental weight on the other, to form a counterpoise. This is effected by the man’s bending, and leaning on the side not loaded, so as to form an equilibrium to the accidental weight he carries; and this cannot be done, unless the loaded shoulder be raised, and the other lowered. This is the resource with which Nature has furnished a man on such occasions.
Anyfigure bearing an additional weight out of the central line, must throw as much natural or accidental weight on the opposite side as is sufficient to form a counterpoise round that line, which passes from the pit of the neck, through the whole mass of weight, to that part of the foot which rests upon the ground. We observe, that when a man lifts a weight with one arm, he naturally throws out the opposite arm; and if that be not enough to form an equipoise, he will add as much of his own weight, by bending his body, as will enable him to resist such accidental load. We see also, that a man ready to fall sideways and backwards at the same time, always throws out the arm on the opposite side.
Whethera man moves with velocity or slowness, the parts above the leg which sustains the weight, will always be lower than the others on the opposite side.
Theshoulders or sides of a man, or any other animal, will preserve less of their level, in proportion to the slowness of their motion; and,vice versâ, those parts will lose less of their level when the motion is quicker. This is proved by the ninth proposition, treating of local motions, where it is said, any weight will press in the direction of the line of its motion; therefore the whole moving towards any one point, the parts belonging to it will follow the shortest line of the motion of its whole, without giving any of its weight to the collateral parts of the whole.
Plate 11Page 35.Chap. 84.Plate 11.London, Published by J. Taylor High Holborn.
Page 35.Chap. 84.Plate 11.London, Published by J. Taylor High Holborn.
Page 35.
Chap. 84.
Plate 11.
London, Published by J. Taylor High Holborn.
Plate 12Page 35.Chap. 84.Plate 12.London, Published by J. Taylor High Holborn.
Page 35.Chap. 84.Plate 12.London, Published by J. Taylor High Holborn.
Page 35.
Chap. 84.
Plate 12.
London, Published by J. Taylor High Holborn.
Plate 13Page 35.Chap. 85.Plate 13.London, Published by J. Taylor High Holborn.
Page 35.Chap. 85.Plate 13.London, Published by J. Taylor High Holborn.
Page 35.
Chap. 85.
Plate 13.
London, Published by J. Taylor High Holborn.
Ithas been objected, in regard to the first part of the above proposition, that it does not follow that a man standing still, or moving slowly, has his members always in perfect balance upon the centre of gravity; because we do not find that Nature always follows that rule, but, on the contrary, the figure will sometimes bend sideways, standing upon one foot; sometimes it will rest part of its weight upon that leg which is bent at the knee, as is seen in the figures B C. But I shall reply thus, that what is not performed by the shoulders in the figure C, is done by the hip, as is demonstrated in another place.
Inthe same proportion as that part of the naked figure marked D A, lessens in height from the shoulder to the hip, on account of its position the opposite side increases. And this is the reason: the figure resting upon one (suppose the left) foot, that foot becomes the centre of all the weight above; and the pit of the neck, formed by the junction of the two Clavicles, quits also its natural situation at the upper extremityof the perpendicular line (which passes through the middle surface of the body), to bend over the same foot; and as this line bends with it, it forces the transverse lines, which are always at right angles, to lower their extremities on that side where the foot rests, as appears in A B C. The navel and middle parts always preserve their natural height.
Inthe bending of the joints it is particularly useful to observe the difference and variety of shape they assume; how the muscles swell on one side, while they flatten on the other; and this is more apparent in the neck, because the motion of it is of three sorts, two of which are simple motions, and the other complex, participating also of the other two.
The simple motions are, first, when the neck bends towards the shoulder, either to the right or left, and when it raises or lowers the head. The second is, when it twists to the right or left, without rising or bending, but straight, with the head turned towards one of the shoulders. The third motion, which is called complex, is, when to the bending of it is added the twisting, as when the ear leans towards one of the shoulders, the head turning the same way, and the face turned upwards.
Ofthose which the shoulders can perform, simple motions are the principal, such as moving the arm upwards and downwards, backwards and forwards. Though one might almost call those motions infinite, for if the arm can trace a circle upon a wall, it will have performed all the motions belonging to the shoulders. Every continued quantity being divisiblead infinitum, and this circle being a continued quantity, produced by the motion of the arm going through every part of the circumference, it follows, that the motions of the shoulders may also be said to be infinite.
Plate 14Page 37.Chap. 89.Plate 14.London, Published by J. Taylor High Holborn.
Page 37.Chap. 89.Plate 14.London, Published by J. Taylor High Holborn.
Page 37.
Chap. 89.
Plate 14.
London, Published by J. Taylor High Holborn.
Whenyou mean to represent a man removing a weight, consider that the motions are various, viz. either a simple motion, by bending himself to raise the weight from the ground upwards, or when he drags the weight after him, or pushes it before him, or pulls it down with a rope passing through a pulley. It is to be observed, that the weight of the man’s body pulls the more in proportion as the centre of his gravity is removed from the centre of his support. To this must be added the strength of the effort that the legs and back make when they are bent, to return to their natural straight situation.
A man never ascends or descends, nor walks at all in any direction, without raising the heel of the back foot.
Whena man prepares himself to strike a violent blow, he bends and twists his body as far as he canto the side contrary to that which he means to strike, and collecting all his strength, he, by a complex motion, returns and falls upon the point he has in view[21].
A manthrowing a dart, a stone, or any thing else with violence, may be represented, chiefly, two different ways; that is, he may be preparing to do it, or the act may be already performed. If you mean to place him in the act of preparation, the inside of the foot upon which he rests will be under the perpendicular line of the pit of the neck; and if it be the right foot, the left shoulder will be perpendicular over the toes of the same foot.
Hewho wishes to pitch a pole into the ground, or draw one out of it, will raise the leg and bend the knee opposite to the arm which acts, in order to balance himself upon the foot that rests, without which he could neither drive in, nor pull out any thing.
Ofthe two arms, that will be most powerful in its effort, which, having been farthest removed from its natural situation, is assisted more strongly by the other parts to bring it to the place where it means to go. As the man A, who moves the arm with a club E, and brings it to the opposite side B, assisted by the motion of the whole body.
Plate 15Page 38.Chap. 90.Plate 15.London, Published by J. Taylor High Holborn.
Page 38.Chap. 90.Plate 15.London, Published by J. Taylor High Holborn.
Page 38.
Chap. 90.
Plate 15.
London, Published by J. Taylor High Holborn.
Plate 16Page 39.Chap. 92.Plate 16.London, Published by J. Taylor High Holborn.
Page 39.Chap. 92.Plate 16.London, Published by J. Taylor High Holborn.
Page 39.
Chap. 92.
Plate 16.
London, Published by J. Taylor High Holborn.
Naturewill of itself, and without any reasoning in the mind of a man going to jump, prompt him to raise his arms and shoulders by a sudden motion, together with a great part of his body, and to lift them up high, till the power of the effort subsides. This impetuous motion is accompanied by an instantaneous extension of the body which had bent itself, like a spring or bow, along the back, the joints of the thighs, knees, and feet, and is let off obliquely, that is, upwards and forwards; so that the disposition of the body tending forwards and upwards, makes it describe a great arch when it springs up, which increases the leap.
Whena man jumps upwards, the motion of the head is three times quicker than that of the heel, before the extremity of the foot quits the ground, and twice as quick as that of the hips; because three angles are opened and extended at the same time: the superior one is that formed by the body at its joint with the thigh before, the second is at the joint ofthe thighs and legs behind, and the third is at the instep before[22].
Inregard to the freedom and ease of motions, it is very necessary to observe, that when you mean to represent a figure which has to turn itself a little round, the feet and all the other members are not to move in the same direction as the head. But you will divide that motion among four joints, viz. the feet, the knees, the hips, and the neck. If it rests upon the right leg, the left knee should be a little bent inward, with its foot somewhat raised outward. The left shoulder should be lower than the other, and the nape of the neck turned on the same side as the outward ankle of the left foot, and the left shoulder perpendicular over the great toe of the right foot. And take it as a general maxim, that figures do not turn their heads straight with the chest, Nature having for our convenience formed the neck so as to turn with ease on every side, when the eyes want to look round; and to this the other joints are in some measure subservient. If the figure be sitting, and the arms have some employment across the body, the breast will turn over the joint of the hip.
Plate 17Page 41.Chap. 96.Plate 17.London, Published by J. Taylor High Holborn.
Page 41.Chap. 96.Plate 17.London, Published by J. Taylor High Holborn.
Page 41.
Chap. 96.
Plate 17.
London, Published by J. Taylor High Holborn.
Thegreatest twist that the body can perform is when the back of the heels and the front of the face are seen at the same time. It is not done without difficulty, and is effected by bending the leg and lowering the shoulder on that side towards which the head turns. The cause of this motion, and also which of the muscles move first and which last, I shall explain in my treatise on anatomy[23].
Itis impossible to turn the leg inwards or outwards without turning the thigh by the same motion, because the setting in of the bones at the knee is such, that they have no motion but backwards and forwards, and no more than is necessary for walking or kneeling; never sideways, because the form of the bones at the joint of the knee does not allow it.If this joint had been made pliable on all sides, as that of the shoulder, or that of the thigh bone with the hip, a man would have had his legs bent on each side as often as backwards and forwards, and seldom or never straight with the thigh. Besides, this joint can bend only one way, so that in walking it can never go beyond the straight line of the leg; it bends only forwards, for if it could bend backwards, a man could never get up again upon his feet, if once he were kneeling; as when he means to get up from the kneeling posture (on both knees), he gives the whole weight of his body to one of the knees to support, unloading the other, which at that time feels no other weight than its own, and therefore is lifted up with ease, and rests his foot flat upon the ground; then returning the whole weight upon that foot, and leaning his hand upon his knee, he at once extends the other arm, raises his head, and straightening the thigh with the body, he springs up, and rests upon the same foot, while he brings up the other.
Figuresthat are set in a fixed attitude, are nevertheless to have some contrast of parts. If one arm come before, the other remains still or goes behind. If the figure rest upon one leg, the shoulder on that side will be lower than the other. This is observed by artists of judgment, who always take care to balance the figure well upon its feet, for fear it shouldappear to fall. Because by resting upon one foot, the other leg, being a little bent, does not support the body any more than if it were dead; therefore it is necessary that the parts above that leg should transfer the centre of their weight upon the leg which supports the body.
Themembers are to be suited to the body in graceful motions, expressive of the meaning which the figure is intended to convey. If it had to give the idea of genteel and agreeable carriage, the members must be slender and well turned, but not lean; the muscles very slightly marked, indicating in a soft manner such as must necessarily appear; the arms, particularly, pliant, and no member in a straight line with any other adjoining member. If it happen, on account of the motion of the figure, that the right hip be higher than the left, make the joint of the shoulder fall perpendicularly on the highest part of that hip; and let that right shoulder be lower than the left. The pit of the neck will always be perpendicular over the middle of the instep of the foot that supports the body. The leg that does not bear will have its knee a little lower than the other, and near the other leg.
In regard to the positions of the head and arms, they are infinite, and for that reason I shall not enter into any detailed rule concerning them; suffice it to say, that they are to be easy and free, graceful, andvaried in their bendings, so that they may not appear stiff like pieces of wood.
Itis impossible that any memory can be able to retain all the aspects or motions of any member of any animal whatever. This case we shall exemplify by the appearance of the hand. And because any continued quantity is divisiblead infinitum, the motion of the eye which looks at the hand, and moves from A to B, moves by a space A B, which is also a continued quantity, and consequently divisiblead infinitum, and in every part of the motion varies to its view the aspect and figure of the hand; and so it will do if it move round the whole circle. The same will the hand do which is raised in its motion, that is, it will pass over a space, which is a continued quantity[24].
Moving Fingers
Neverput the head straight upon the shoulders, but a little turned sideways to the right or left, even though the figures should be looking up or down, or straight, because it is necessary to give them some motion of life and spirit. Nor ever compose a figure in such a manner, either in a front or back view, as that every part falls straight upon another from the top to the bottom. But if you wish to introduce such a figure, use it for old age. Never repeat the same motion of arms, or of legs, not only not in the same figure, but in those which are standing by, or near; if the necessity of the case, or the expression of the subject you represent, do not oblige you to it[25].
Thevariety of motions in man are equal to the variety of accidents or thoughts affecting the mind, and each of these thoughts, or accidents, will operate more or less, according to the temper and age of the subject; for the same cause will in the actions of youth, or of old age, produce very different effects.
Simplemotion is that which a man performs in merely bending backwards or forwards.
Complexmotion is that which, to produce some particular action, requires the body to bend downwards and sideways at the same time. The painter must be careful in his compositions to apply these complex motions according to the nature of the subject, and not to weaken or destroy the effect of it by introducing figures with simple motions, without any connexion with the subject.
Themotions of your figures are to be expressive of the quantity of strength requisite to the force of the action. Let not the same effort be used to take up a stick as would easily raise a piece of timber. Therefore shew great variety in the expression of strength, according to the quality of the load to be managed.
Thereare some emotions of the mind which are not expressed by any particular motion of the body, while in others, the expression cannot be shewn without it. In the first, the arms fall down, the hands and all the other parts, which in general arethe most active, remain at rest. But such emotions of the soul as produce bodily action, must put the members into such motions as are appropriated to the intention of the mind. This, however, is an ample subject, and we have a great deal to say upon it. There is a third kind of motion, which participates of the two already described; and a fourth, which depends neither on the one nor the other. This last belongs to insensibility, or fury, and should be ranked with madness or stupidity; and so adapted only to grotesque or Moresco work.
Itis not becoming in women and young people to have their legs too much asunder, because it denotes boldness; while the legs close together shew modesty.
Childrenand old people are not to express quick motions, in what concerns their legs.
Letevery member be employed in performing its proper functions. For instance, in a dead body, or one asleep, no member should appear alive or awake. A foot bearing the weight of the whole body, should not be playing its toes up and down, but flat upon the ground; except when it rests entirely upon the heel.
A merethought, or operation of the mind, excites only simple and easy motions of the body; not this way, and that way, because its object is in the mind, which does not affect the senses when it is collected within itself.
Whenthe motion is produced by the presence of some object, either the cause is immediate or not. If it be immediate, the figure will first turn towards it the organs most necessary, the eyes; leaving its feet in the same place; and will only move the thighs, hips, and knees a little towards the same side, to which the eyes are directed.
Thosewho become enamoured of the practice of the art, without having previously applied to the diligent study of the scientific part of it, may be compared to mariners, who put to sea in a shipwithout rudder or compass, and therefore cannot be certain of arriving at the wished-for port.
Practice must always be founded on good theory; to this, Perspective is the guide and entrance, without which nothing can be well done.
Perspectiveis to Painting what the bridle is to a horse, and the rudder to a ship.
The size of a figure should denote the distance at which it is situated.
If a figure be seen of the natural size, remember that it denotes its being near to the eye.
Theoutlines or contours of bodies are so little perceivable, that at any small distance between that and the object, the eye will not be able to recognise the features of a friend or relation, if it were not for their clothes and general appearance. So that by the knowledge of the whole it comes to know the parts.
LinearPerspective consists in giving, by established rules, the true dimensions of objects, according to their respective distances; so that the second object be less than the first, the third than the second, and by degrees at last they become invisible. I find by experience, that, if the second object be atthe same distance from the first, as the first is from the eye, though they be of the same size, the second will appear half the size of the first; and, if the third be at the same distance behind the second, it will diminish two thirds; and so on, by degrees, they will, at equal distances, diminish in proportion; provided that the interval be not more than twenty cubits[26]; at which distance it will lose two fourths of its size: at forty it will diminish three fourths; and at sixty it will lose five sixths, and so on progressively. But you must be distant from your picture twice the size of it; for, if you be only once the size, it will make a great difference in the measure from the first to the second.
Thoseparts which are of less magnitude will first vanish from the sight[27]. This happens, because the shape of small objects, at an equal distance, comes to the eye under a more acute angle than the large ones, and the perception of them is less, in proportion as they are less in magnitude. It follows then, that if the large objects, by being removed to a great distance, and consequently coming to the eye by a small angle, are almost lost to the sight, the small objects will entirely disappear.
Theoutlines of objects will be less seen, in proportion as they are more distant from the eye.
Thepoint of sight must be on a level with the eyes of a common-sized man, and placed upon the horizon, which is the line formed by a flat country terminating with the sky. An exception must be made as to mountains, which are above that line.
Thiswill be proved by one single example. If you mean to represent a round ball very high up, on a flat and perpendicular wall, it will be necessary to make it oblong, like the shape of an egg, and to place yourself (that is, the eye, or point of view) so far back, as that its outline or circumference may appear round.
Thefirst figure in your picture will be less than Nature, in proportion as it recedes from the front of the picture, or the bottom line; and by the same rule the others behind it will go on lessening in an equal degree[28].
Thefirst things that disappear, by being removed to some distance, are the outlines or boundaries of objects. The second, as they remove farther, are the shadows which divide contiguous bodies. The third are the thickness of legs and feet; and so in succession the small parts are lost to the sight, till nothing remains but a confused mass, without any distinct parts.
Supposingthis small object to represent a man, or any other animal, although the parts, by being so much diminished or reduced, cannot be executed with the same exactness of proportion, nor finished with the same accuracy, as if on a larger scale, yet on that very account the faults will be less conspicuous. For example, if you look at a man at the distance of two hundred yards, and with all due attention mean to form a judgment, whether he be handsome or ugly, deformed or well made, you will find that, with all your endeavours, you can hardly venture to decide. The reason is, that the man diminishes so much by the distance, that it is impossibleto distinguish the parts minutely. If you wish to know by demonstration the diminution of the above figure, hold your finger up before your eye at about nine inches distance, so that the top of your finger corresponds with the top of the head of the distant figure: you will perceive that your finger covers, not only its head, but part of its body; which is an evident proof of the apparent diminution of that object. Hence it often happens, that we are doubtful, and can scarcely, at some distance, distinguish the form of even a friend.
Thiscustom, which has been generally adopted by painters, on the front and sides of chapels, is much to be condemned. They begin with an historical picture, its landscape and buildings, in one compartment. After which, they raise another compartment, and execute another history with other buildings upon another level; and from thence they proceed to a third and fourth, varying the point of sight, as if the beholder was going up steps, while, in fact, he must look at them all from below, which is very ill judged in those matters.
We know that the point of sight is the eye of the spectator; and if you ask, how is a series of subjects, such as the life of a saint, to be represented, in different compartments on the same wall? I answer, that you are to place the principal event in the largest compartment, and make the point ofsight as high as the eye of the spectator. Begin that subject with large figures; and as you go up, lessen the objects, as well the figures, as buildings, varying the plans according to the effect of perspective; but never varying the point of sight: and so complete the series of subjects, till you come to a certain height, where terrestrial objects can be seen no more, except the tops of trees, or clouds and birds; or if you introduce figures, they must be aerial, such as angels, or saints in glory, or the like, if they suit the purpose of your history. If not, do not undertake this kind of painting, for your work will be faulty, and justly reprehensible[29].
Paintersoften despair of being able to imitate Nature, from observing, that their pictures have not the same relief, nor the same life, as natural objects have in a looking-glass, though they both appear upon a plain surface. They say, they have colours which surpass in brightness the quality of the lights,and in darkness the quality of the shades of the objects seen in the looking-glass; but attribute this circumstance to their own ignorance, and not to the true cause, because they do not know it. It is impossible that objects in painting should appear with the same relief as those in the looking-glass, unless we look at them with only one eye.
The reason is this. The two eyes A B looking at objects one behind another, as M and N, see them both; because M cannot entirely occupy the space of N, by reason that the base of the visual rays is so broad, that the second object is seen behind the first. But if one eye be shut, and you look with the other S, the body F will entirely cover the body R, because the visual rays beginning at one point, form a triangle, of which the body F is the base, and being prolonged, they form two diverging tangents at the two extremities of F, which cannot touch the body R behind it, therefore can never see it[30].