David Hartley

“ For, when we view any object with attention, we make the central point of it fall upon the central part of the retina. Farther, as the optic nerve sends off no branches, but is spent wholly upon the retina, this may perhaps contribute in some degree. And these considerations may a little help us to conceive, how the optic nerve, and corresponding region of the brain, may be the repository of such an immense variety of visible ideas, as they are in fact. ”

“ A large object, when apprehended to be one of a common size, appears nearer than the truth; and the same happens, when we view objects in rural scenes, such as houses, towns, hills, &c. in a bright light, or through a very clear atmosphere. In trying to judge of small distances by one eye, it is usual to be mistaken for want of the criterion from the inclination of the optic axes. ”

“ As therefore the external influence of light, by growing languid, comes to have less and less effect perpetually for this purpose, so the associated tendency grows stronger and stronger; and the sum total of both may perhaps be always about equal.
However, this congruity does not seem to be so great in children as in adults, who can direct their optic axes by a perfectly voluntary power to the object which they intend to view with attention. We must examine therefore, how the almost perfect congruity becomes an entirely perfect one. ”

“ In adults, the pleasures of mere colours are very languid in comparison of their present aggregates of pleasure, formed by association. And thus the eye approaches more and more, as we advance in spirituality and perfection, to an inlet for mental pleasure, and an organ suited to the exigencies of a being, whose happiness consists in the improvement of his understanding and affections. ”

“ And thus an impression made on the right eye alone by a single object may propagate itself into the left, and there raise up an image almost equal in vividness to itself: and consequently, when we see with one eye only, we may, however, have pictures in both eyes; and when we see a single object, with our eyes directed to one at a different distance, we may have four pictures, viz. two from direct impression in parts that do not correspond, and two others from association in parts that do. ”

“ Sometimes, instead of a luminous image, a dark one, corresponding in shape and size to the luminous object, presents itself. In this case we must suppose, that the vivid vibrations excited by the luminous object pass immediately into very feeble ones, absolutely or relatively, upon the cessation of the impression.
Boerhaave says, that he had a luminous circle in his eye for a long time after having viewed too intensely the moon’s light collected to a focus. And looking at the sun makes other objects appear red and luminous. ”

“ For natural bodies reflect all these colours in great abundance, and in sufficient purity for this purpose. We may begin from green, as the most common of all. When this, as reflected by grass, suppose, has been sufficiently familiarized to the eye of a child, it is reasonable to think, that it may be distinguished from yellow and blue, and much more from red and violet, as reflected by flowers ”

“ We judge of motion by the motion of the pictures on the retina, or of our eyes in following the objects. After some time, we learn to make allowance for the line of direction, our own motions, &c. If we fail to make the due allowance through associated circumstances of any kind, we must, in consequence of this, make a disproportionate estimate of motion, or place it in an undue object. ”

“ And yet the horizontal moon appears far less than the truth, because we can form no conception of its vast distance. A tree referred to the horizon in the dusk of the evening, or a fly to the ground at a distance, through the indistinctness of vision, appears much bigger than the truth. In looking through glasses, which magnify or diminish the picture on the retina, the objects themselves seem to be magnified or diminished, because our judgment concerning the distance is not altered proportionally, &c. ”

“ Thus, if a child be laid so into his cradle, as that one eye shall be covered, the external influences of light cannot operate upon it. And if this be often repeated, especially while the association which confirms the congruity of the motions is weak, the eye which is covered will obey the influences which descend from the brain, and turn upwards and inwards for the most part. ”

“ It happens also frequently, that in the whole appearance a blue, a dilute yellow, and a red, succeed each other in the order of the colours. Perhaps, by farther observations, a person might be able, in some measure, to predict the variations of this phænomenon. It generally moves, which may be perhaps from the motion of the peculiar vibrations along the surface of the retina.
Upon shutting one’s eyes after they have been fixed upon a luminous object, as a candle, a fire, a window, it is common to have a faint image of the object remain in the eye for a few moments. ”

“ It may be expected, that the immediate organ of sight should be either black or white, that so it may bear a relation of indifference to all the colours. But if we admit the doctrine of vibrations, black, by absorbing all kinds of rays, would make a confusion of vibrations, whereas white, by reflecting all, might retain the impressed vibrations distinctly. The retina is therefore peculiarly fitted for the immediate organ of sight, and the choroides the contrary. We may add farther, that the retina, by reflecting rays copiously, prevents their arrival at the choroides. ”

“ Twelfthly, The ideas which different persons have of the same persons’ faces, though they be very like one another, cannot yet be precisely the same, on account of the addition and omission of little circumstances, and a variety of associated ones, which intermix themselves here. Hence the same picture may appear much more like to one person than to another, viz. according as it resembles his idea more or less. ”

“ First, then, When a person directs his eyes by a voluntary power to a point nearer or farther off than the object which he views, so as to make the pictures of the object fall upon the points of the two retinas, that do not correspond, this object will appear double. The same thing happens when one eye is distorted by a spasm, when persons lose the voluntary power of directing their optic axes to objects, and in general whenever the pictures, which the object imprints on the two retinas, fall upon points that do not correspond. ”

“ But the five following associated circumstances seem to have also some influence on our judgments concerning distance, in certain cases, and under certain limitations: the number of objects which intervene, the degree of distinctness in which the minute parts are seen, the degree of brightness, the inclination of the optic axes, and the conformation of the eye. It will appear from the sixty-second and sixty-third propositions that the two last are associates to each other in their proper degrees, since each depends on the distance of the object. ”

“ What excites the first to constant action during the whole time that the new-born child is awake, is difficult to say. Perhaps the action of light upon the white of the eye, sending vibrations under the upper eyelid, and thence into the fibres of the elevator: or the direct action of light through the skin; for it is a cutaneous muscle: or influences which descend into the third pair from the second, i.e. the optic nerve: or the friction from the globe of the eye in its motions, which may also make the eyelid sympathize in motion with the eye: or the aggregate of all these. ”

“ The persons who squint, preserving the sight of the squinting eye, are obliged to move their eyes in a congruous manner, for the same reason as others, viz. to avoid double vision, though the position be incongruous.
The constant action of the elevator palpebræ superioris in adults seems to be entirely from custom, i.e. association, being kept up in a more particular manner by the variety of visible objects, which engage our attention during the course of the day. ”

“ Our calling bodies erect, when the rays proceeding from their tops fall upon the lower parts of the retina, and vice versâ, is merely from an association of the same kind with those by which the senses of other words are determined. ”

“ Since our judgment concerning the magnitude of an unknown object depends upon the distance, and our judgment concerning the distance of every object chiefly upon that concerning its magnitude, the conjectures of different persons, concerning the magnitudes and distances of unknown remote objects, both as seen through telescopes, and with the naked eye, may vary considerably from each other, according to their respective associated prejudices. ”

“ As to the radiated fibres of the iris, and the ciliar ligament, they do not seem so much to be excited to action by any external influence, as to be kept in a state of constant small activity by the vibrations which descend from the brain. When therefore the rings are relaxed, the radiated fibres will open the pupil, and the ciliar ligament draw out the capsula of the crystalline to a flatter shape, and thus suit the eye to obscure and distant objects. ”

“ Adults seem to have a power of preparing the eyes previously to see at a proposed distance; and some to have a semi-voluntary power of contracting and dilating the pupil, viz. by fancying a bright object near, and a dark one far off; though the quantity of light which falls upon the eye remains the same. ”

“ To which we may add, that various luminous objects generally affect the eyes at the same time; that the four straight muscles do naturally balance each other, and keep the eyes in a right forward position; and that the oblique muscles scarce favour any particular oblique position, though they do prepare the eye to turn with greater facility, in compliance with the contraction of any one of the four straight muscles. There are therefore sufficient sources for a variety of motions in the globes of the eyes, without destroying their congruity. ”

“ We judge of the figure or shape of bodies, chiefly by the variations of light and shade; and our associations taken thence are so strong, as that we are easily imposed upon by a just imitation of the light and shades belonging to each shape and figure, in their several situations with respect to the quarter from which the illumination proceeds. ”

“ Ninthly, The trains of visible ideas are in a particular manner affected by the general states of the brain, as may appear from the trains which present themselves in madness, phrensies, and common deliriums. ”

“ However, the introduction of the idea of a horse, in the circumstances just decribed, might be termed voluntary in a different sense, if any person thought fit to denominate it so, on account of the command of the will to introduce some idea. ”

“ If there be, it must, however, admit of much greater latitude than the harmony between sounds, since all mixtures and degrees of colours, unless where the quantity of light overpowers the eye, are pleasant; however one colour may be more so originally than another. Black appears to be originally disagreeable to the eyes of children ”

“ Thus a person of an ordinary height standing very near a very tall one, or coming in at a very high door, appears shorter than the truth; lean persons seem tall, fat persons short, &c.
The principal criterion of distance is the magnitude of the picture, which some known object makes on the retina. ”

“ The ghastly faces which sometimes appear in idea, particularly after drinking tea, seem to be an effect of this kind, or perhaps of the last-mentioned one; for they are common to persons of irritable nervous systems. Ghastly faces may take place preferably to other disagreeable ideas, perhaps because characters, affections, passions, are principally denoted and expressed by the countenance; because faces are the most common of visible objects, and attended to with the greatest earnestness ”

“ The trains of visible ideas, which occur in dreams, are deducible, partly from the sympathy here mentioned, partly from that of the last paragraph.
Eleventhly, Our stock of visible ideas may be considered as a key to a great part in our knowledge, and a principal source of invention in poetry, painting, mathematics, mechanics, and almost every other branch of the arts and sciences. In mathematics and mechanics the invention of the diagram is, in effect, the solution of the problem. ”

“ Secondly, After a person, whose eye is distorted by a spasm, has seen double for a certain time, this ceases, and he gains the power of seeing single again, provided the distortion remain fixed to a certain degree. ”

“ Fourthly, But it is not necessary, that the eyes of new-born children should always turn to the luminous object, or remain fixed upon it. For every muscle, when it has exhausted itself by contraction, gives place to its antagonist of course. If therefore the luminous object be much on one side, the eyes ought to turn back from it almost immediately. ”

“ But when the vibrations of the extreme rays are greatly different from each other, it seems that each ought to keep the power of exciting its proper vibrations, so as to make the colour of the middle rays ”