Observations on Man (6th edition)/Part I/Chapter II/Section IV
Section IV
[edit]THE SENSE OF SIGHT.
Since the retina is an expansion of the optic nerve, we may conclude, from the analogy of the other senses, that it is the immediate organ of sight. Nor is the want of sensibility in the button of the optic nerve a sufficient objection to this; as the minute structure and disposition of the parts of this button are not known.
We may also reason thus to the same purpose. 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.
For the accurate distinction of the several visible points of objects, it is necessary that these be placed within the limits of distinct vision; and also, that the coats and humours of the eye be so circumstanced, as to bring the several pencils of rays, which proceed from each visible point accurately or nearly, to a corresponding point upon the retina. This is distinct vision. But colours alone may be distinguished from each other without any exact conformation of the eye. Thus vision may be reckoned of two kinds, as feeling, taste, and smell, have been.
Here I will make two suppositions.
First, That the extreme red rayss at F M, Optics, book I. part 2. fig. 4. excite vibrations in the retina, which are to those excited by the extreme violet rays at A G, as 1 to 2, in respect of frequency.
Secondly, That in going from the extreme red to the extreme violet, the excess of vibrations excited by each colour, above those of the extreme red, will be proportional to its distance from the extreme red.
If we admit these two suppositions, then the vibrations excited by the extreme red, by the limit of red and orange, of orange and yellow, yellow and green, green and blue, blue and indigo, indigo and violet, and by the extreme violet, as these colours are fixed by Sir Isaac Newton, will be to one another in frequency, respectively as the 8 numbers the distances of these several limits, and of the extreme violet, from the extreme red, being to one another respectively, as the 7 numbers
Now the first suppositions may be rendered probable thus. The intervals of the fits of easy reflection and transmission of the red and violet in the same medium, and same angle of refraction, are nearly as 5 to 3. See Optics, book II. Obs. 13, 14, and Prop. XVI. But the red is less refracted by the coats and humours of the eye than the violet, and consequently will not have its intervals so much diminished in proportion; whence they may be to those of the violet as 6 to 3, or 2 to 1, at their arrival on the retina. But it is probable, that the vibrations of the rays themselves, and consequently those which they excite in the retina, are reciprocally as the intervals of their fits. The frequency therefore of the vibrations excited by the extreme red may be to that of the vibrations excited by the extreme violet as 1 to 2, according to the first supposition.
The second supposition is an easy step after the first.
For it is natural to suppose, that in passing from F to A, in the figure above referred to, equal distances should produce an equal increase of vibrations, which is the second supposition.
Upon this foundation we may now reason in the following manner.
First, The seven primary colours, estimated both from their limits, and their middle points, excite vibrations, which are to each other in the simplest ratios that are consistent with each other, and all comprehended within the first and most simple of all ratios, viz. that expressed by the two first numbers 1 and 2.
Secondly, The same ratios are also those of the five tones, and two semi-tones, comprehended within the octave; as might well be expected. For music must take those which are most simple, and most consistent with each other.
Thirdly, Since the greens are respectively to the yellows, on one hand, as 9 to 8, and to the blues on the other, as 9 to 10, i.e. in the proportion of a tone; also to the reds, on one hand, as 4 to 3, and to the violets, on the other, as 3 to 4, i.e. in the proportion of a 4th; since farther, the yellows are as 6 to 5, i.e. thirds minor, to the reds, as 4 to 5, i.e. thirds major, to the blues, and as 2 to 3, i.e. fifths, to the violets; the blues as 5 to 6, i.e. thirds minor, to the violets, and as 3 to 2, i.e. fifths, to the reds; and the reds as 9 to 16, i.e. flat sevenths, to the violets; the difference of vibrations here exhibited may make the five foregoing colours appear distinct from each other to the mind, for the same reasons, whatever they be, as take place in sounds. 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; also that these may be distinguished from each other. And it seems to me, that our fixed point ought to be placed in green, from the commonness and purity of the green of the third order, i.e. of grass and vegetables in general. For the same reasons one may expect, that the several shades of red, orange, green, blue, and violet, should be considered as several degrees of the same colour, viz. on account of the small difference of vibrations. At least this corresponds to the usual method of proceeding in other things. We distinguish great differences in our sensations by new names; but refer all such as are nearly related to the same. And thus the two foregoing suppositions furnish us with a natural reason for distinguishing the primary colours into five, viz. red, yellow, green, blue, and violet; which, agreeably to this, were all that Sir Isaac Newton himself distinguished the oblong solar image into for some time, as may appear by his optical lectures.
Fourthly, Since, if we proceed from the green to the yellow and red, on one hand, and to the blue and violet on the other, the ratios are the same, only inverted; and since there is a larger interval or ratio between the yellow and red, also between the blue and violet, than between the green and yellow, or green and blue; we may expect to have two more distinct primary colours corresponding to each other, and to the two semi-tones in an octave. And thus it is. Orange, and indigo, are sufficiently distinct from their contiguous ones, viz. orange from red and yellow, and indigo from blue and violet; and yet approach to them. And these seven colours, thus fixed, seem to be all that we can well call distinct colours amongst the primary ones, the intermediate degrees being referred to some of these seven, and called shades. Of compound colours, distinct from all the primary ones, I shall speak below.
Fifthly, It is remarkable here, that the order of the five tones and two semi-tones of an octave, which corresponds to the order of the seven primary colours, is the second in absolute perfection (which I have from a MS. paper of Sir Isaac Newton’s on music, not yet published), and the first in relative, i.e. of those, in which the semi-tones are at equal distances from the middle or extremes; which circumstance is evidently necessary in the order of the colours. For if distinct colours arise from ratios, and a half-note colour arise next after the red, if you begin at one end, a corresponding one ought to appear next after the violet, if you begin at the other. The sameness of the ratios that must arise, makes this necessary, on supposition, that the distinction of colours is founded on ratios.
Sixthly, If the distinction of colours arise from the ratios of vibrations, the colours may be expected to be broader where the vibrations are more numerous, because a greater addition must be made to a greater number, in order to make an equal ratio. And there is a certain breadth for each of the colours respectively, which suits each set of ratios of vibrations that they can be supposed to bear to one another, according to any supposed law of increase of the vibrations in passing from one end of the solar image to the other. Since therefore the breadth of the seven primary colours, as determined by Sir Isaac Newton, suits the simplest ratios possible, according to the simplest law of increase possible, as has been explained above, we seem to have from thence an argument both for the doctrine of vibrations in general, and for the particular ratios of vibrations here alleged. And there are two things in this matter which deserve particular notice. First, that Sir Isaac Newton’s Spectrum was about ten inches long; and consequently, the breadths of the seven primary colours, red, orange, yellow, green, blue, indigo, violet, in inches, 1.25; 0.75; 1.33; 1.66; 1.66; 1.11; 2.22; which magnitudes are so considerable, that a small error in fixing the limit of a colour does not much affect their mutual ratios. Secondly, that the limits of the colours were determined in a way, that had no dependence on any hypothesis, and the operation repeated several times. However, it may perhaps be worth the time and pains of some curious experimenter, to examine the breadths of the seven primary colours afresh, and compare them with the hypothesis here proposed.
Seventhly, When all the rays reflected from any natural body are near to each other, as in the yellows of the second order, and in the blues and greens of the third, we may suppose, that the slower vibrations are accelerated by the quicker, and the quicker retarded by the slower, so as to compose an intermediate colour, scarce differing from homogeneal light in appearance of purity; just as in a bell, the slower vibrations of the wider part, and the quicker of the narrower, overrule each other mutually, so as to compose one tone. 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; which may be considered as a kind of centre of gravity, a dilute one, verging to white. And white itself, when in perfection, arises from a due proportion of all the sorts of rays, each primary colour, perhaps, keeping its own peculiar vibrations, and the several shades of each primary colour vibrating in the same time as the middle point. When two colours considerably different, as red and blue, yellow and violet, red and violet, are compounded, they neither resemble the intermediate homogeneal one, nor make a white. Not the first, because they are at so great a distance, that each can keep its own vibrations, contrary to what happens in colours resembling homogeneal ones; not a white, because there is not a sufficient number of differing vibrations. By such compositions it is, that purples, and other colours, different from all the homogeneal ones, are formed; and whoever considers the several shades of each colour, with the mutual proportions which may be combined in any compound, may easily conceive how all the colours of natural bodies should arise from mere combinations of the primary colours, agreeably to the sixth and seventh propositions, of the second part of the first book of Sir Isaac Newton’s Optics. What is here delivered may serve to suit the doctrine of vibrations to those propositions, and, perhaps, assist the reader to see the reasons of the sixth.
Cor. If the differences of the primary colours arise from the specific differences of vibrations, it is easy to see, that the differences of tastes and smells may have a like origin; and vice versâ.
Flashes of light, and other luminous appearances, are occasioned by strokes upon the eye, rubbing it, faintings, &c. Now it is very easy to conceive, that violent agitations in the small particles of the optic nerve should arise from these causes; and consequently that such deceptions of the sight, as one may call them, should be produced, if we admit the doctrine of vibrations. And I do not see how they follow from the common hypothesis concerning the manner of sensation.
The most remarkable of these luminous appearances is that which resembles the eye of a peacock’s feather, and which offers itself upon shutting and rubbing the eye in a morning. There is a distinction in it between the central parts and the edges. The first seems to answer to that part of the retina, which is opposite to the pupil, and of about the same size with it, in its ordinary dimensions. The last, or the edges, may answer to those parts of the retina, which are only sometimes exposed to the action of light, viz. in dilatations of the pupil. It is observable, that the central parts are often dark, while the edges are luminous; and vice versâ. 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. This follows from the gradual declension of the vibrations excited.
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. It does also occasion dark spots to appear upon common objects afterwards. This last effect may perhaps succeed the first. While extraordinary vibrations, or a disposition to them, continue in the retina, and optic nerve (which may be for a longer time, if a slight inflammation, with the consequent irritability, be produced in the small vessels of the nervous capillaments), the common objects may appear luminous. When these go off, a contrary state may take place, and cause the dark spots to appear. Dark spots of continuance argue, that an injury is done to the retina, and optic nerve. The permanent dark spots, which are sometimes previous to a gutta serena, seem to be of this kind.
It is agreeable to some of the foregoing instances, that being kept much in the dark should enable the persons to see with a very obscure light. In some other cases of a nyctalopia there may perhaps be the first and lowest degree of inflammation in the infinitesimal vessels of the retina, so as to increase the sensibility of the organ without making the exercise of its functions painful.
Giddiness, or an apparent irregular motion in the objects of sight, almost always goes before any general confusion and privation of sense and motion; which is very agreeable to the doctrine of vibrations. For the general disorder in the vibrations in the medullary substance may be expected to be perceived in the optic nerve, and corresponding part of the brain, first and chiefly, on account of the acuteness and precision of the sense of sight. Upon the same principles it is easy to see, how great and unusual agitations of the body, impressions on the stomach, on the olfactory nerves, on the eye, by the quick transition of objects, on the eye and fancy together, by looking down a precipice, &c. should occasion a temporary giddiness.
I have already observed, Prop. XXX. that these judgments are to be esteemed true or false, according as they agree or disagree with those made by the touch.
Now the associates of greater tangible magnitude are a larger picture on the retina, the distance being the same; and a larger distance, the picture being the same. The associates of a less tangible magnitude are the opposites to these. And the associates of the sameness of tangible magnitude are the increase or diminution of the picture on the retina, while the distance is diminished or increased suitably thereto. All this appears from optical considerations. Hence it follows, that where the picture on the retina is of a just size, and also the previous judgment concerning the distance just, our estimate of tangible magnitude by sight will be just likewise. But if the picture on the retina be magnified or diminished by glasses, or our previous judgment concerning the distance be erroneous, our estimate of tangible magnitude will be erroneous in like manner. And whether it be just or erroneous, it is entirely founded on association.
The following instances, among many others, confirm these positions. Young children judge rightly of magnitude only in familiar places, or at small distances. At great distances they always judge the objects to be less than the truth, not having learnt to judge rightly of these distances, and make allowance for them. The generality of adults judge far better of magnitude at great distances on level ground, than from above, or from below, on account of their greater experience in the former case. The horizontal moon appears larger than the meridional, because the picture on the retina is of nearly the same size, and the distance esteemed to be greater. 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. &c.
There are, besides these, some other associated circumstances, which occasionally impose upon us in estimating magnitudes. 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. 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. The influence of the three first, as well as that of the magnitude of the picture on the retina, is evident from the methods of expressing distance in pictures.
From the principles laid down in the last paragraph, we may explain the following fallacies in vision. An object viewed through a perspective appears to be nearer than it is, because the picture on the retina is thereby rendered both larger, and more distinct; but if we invert the perspective, and so diminish the picture, the object will appear farther off. At sea, and on plains, where few or no objects intervene, we judge the distances to be less than the truth; and the contrary happens in scenes diversified with a proper variety of objects. 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.
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. If the distance be fixed previously by a known object, we may afterwards judge of the magnitude of an unknown object thereby. The number of intervening objects, and the inclination of the optic axes, seem to afford considerable assistance in determining distances, where known objects are wanting; the first in large distances, the last in small ones: but the other three inferior criterions above-mentioned, viz. the degree of distinctness, the degree of brightness, and the conformation of the eye, when singly taken, are of small signification.
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.
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.
It is from the associations, considered under this proposition, and particularly in the last paragraph, that painting conveys such exact ideas of shapes, figures, magnitudes, and distances, and the camera obscura of motions also, by means of impressions that proceed from a plane surface.
The position of objects is judged of entirely by the part of the retina on which the rays fall, if we be in an erect posture ourselves. If we be not, we allow for our deviation from it, or make a reference to something judged to be in an erect posture. If we fail in these, errors concerning the position of visible objects must happen. 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.
Those who are disposed to examine the subjects of this and the following proposition with accuracy, may see a large variety of proper instances well explained by Dr. Smith, and Dr. Jurin, in Dr. Smith’s Optics. These gentlemen insist chiefly on optical considerations; but they every where admit the prevalence of association, though it is not always to their purpose to take express notice of it.
I will just remind the reader, that in all the cases of magnitude, distance, motion, figure, and position, the visible idea is so much more vivid and ready than the tangible one, as to prevail over it, notwithstanding that our information from feeling is more precise than that from sight, and the test of its truth. However, if we could suppose a person to be endued with the senses of seeing and hearing, and yet to be destitute of that of feeling, and of the power of moving himself, he might have all the words expressing distances, magnitudes, &c. so much, and so properly, associated with the visible appearances of these, as that, by passing over his ear, they would raise up all the same trains of visible ideas, as in us.
When we have attained a voluntary power over the external motions of our eyes, so as to direct them to objects at pleasure, we always do it in such a manner, as that the same points of objects fall upon correspondent points of the two retinas. And this correspondence between the respective points of the retinas is permanent and invariable. Thus the central points, or those where the optic axes terminate, always correspond; a certain point on the right side of the right retina always corresponds (whatever object we view) to another certain point on the right side of the left retina, equally distant from the centre with it, &c. Hence if the optic axes be directed to the object A, the picture made by it on the right retina corresponds to that made on the left; whereas the impressions made by two similar objects, A and B upon the two retinas, do not correspond. The impressions, therefore, that are made upon portions of the retinas, which do or do not correspond, are the associated criterions of single and double vision. For I here suppose, that the common appearances of a single object, and two similar ones, are respectively called single and double vision.
Let us now inquire into the fallacies which these associated criterions may occasion.
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.
It resembles this, and illustrates it, that if we cross the fingers, and roll a pea between two sides, which are not contiguous naturally, it feels like two peas.
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. For the association between the points of the two retinas, which corresponded formerly, grows weaker by degrees; a new one also between points, that now correspond, takes place, and grows stronger perpetually.
Thirdly, If two lighted candles, of equal height, be viewed at the distance of two or three feet from the eyes, so that the picture of the right-hand candle on the left retina shall correspond to that of the left-hand candle on the right retina, only one image will be produced by these two corresponding pictures. But the two pictures which do not correspond, viz. that of the right-hand candle on the right retina, and that of the left-hand candle on the left retina, will each produce its proper image. See Smith’s Optics, Rem. 526.
But here two questions may be asked; First, Why single objects appear the same to one as to both eyes, allowing for the diminution of brightness, since, in the first case, there is one picture only, in the last two. Ought not every single object to appear single to one eye, and double to both?
Secondly, How can one object appear like two to both eyes, since, however the eyes be directed or distorted, it can make but two pictures, whereas two objects make four, viz. two in each eye?
It is evident, that the difficulty is the same in both these questions. And it seems to be a sufficient answer to allege, that impressions so much alike, and which are so constantly made together, as those upon the corresponding portions of the two retinas, must unite into one entirely in the brain, and produce the same effect in kind, though somewhat different in degree, as one alone. And thus whether we see with one eye or both, hear with one ear or both, the impression, on the common sensory in the brain is the same in kind; and therefore, if the first be called single, the other must also.
But it deserves particular attention here, that the optic nerves of men, and such other animals as look the same way with both eyes, unite in the sella turcica in a ganglion, or little brain, as one may call it, peculiar to themselves; and that the associations between synchronous impressions on the two retinas must be made sooner, and cemented stronger, on this account; also that they ought to have a much greater power over one another’s images, than in any other part of the body. 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. And thus both the foregoing questions may be answered, in a manner that leaves no doubt or hesitation.
It is evident, that gay colours, of all kinds, are a principal source of pleasure to young children; and they seem to strike them more particularly, when mixed together in various ways. Whether there be any thing in colours, which corresponds to the harmony between sounds, may be doubted. 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; it becomes disagreeable also very early from associated influences. 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. However, the original pleasures of mere colours remain, in a small degree, to the last, and those transferred upon them by association with other pleasures (for the influence is in these things reciprocal, without limits) in a considerable one. So that our intellectual pleasures are not only at first generated, but afterwards supported and recruited, in part, from the pleasures affecting the eye; which holds particularly in respect of the pleasures afforded by the beauties of nature, and by the imitations of them, which the arts of poetry and painting furnish us with. And for the same reasons the disagreeable impressions on the eye have some small share in generating and feeding intellectual pains.
It deserves notice here, that green, which is the colour that abounds far more than any other, is the middle one among the primary colours, and the most universally and permanently agreeable to the eye of any other; also, that as the common juice of vegetables is in general green, so that of animals is in general red; the first being, perhaps, of the third order, the last of the second. It appears to be extremely worth the time and pains of philosophers to inquire into the orders of the colours of natural bodies, in the manner proposed and begun by Sir Isaac Newton; and particularly to compare the changes of colour, which turn up in chemical operations, with the other changes which happen to the subjects of the operations at the same time. Nothing seems more likely than this to be a key to the philosophy of the small parts of natural bodies, and of their mutual influences.
Here we may make the following observations: First, that the ideas of this sense are far more vivid and definite than those of any other; agreeably to which, the word idea denoted these alone in its original and most peculiar sense. Hence it is proper to make the strictest examination into the ideas of this sense, and their properties, since it is probable, from the analogies every where conspicuous in natural things, that these are patterns of all the rest. Their peculiar vividness and precision may therefore be considered as serving like a microscope in respect of other ideas, i.e. as magnifying their properties.
Secondly, The vividness and precision here spoken of relate chiefly to distance, magnitude, motion, figure, and position, i.e. to the things considered in the fifty-eighth proposition. However, colours leave distinct ideas of themselves; but then they require an exertion of our voluntary powers for the most part, whereas the ideas of distances, magnitudes, &c. recur incessantly in the trains which pass over the fancy.
Thirdly, The peculiar vividness and precision of visible ideas may probably be owing to the following causes, as well as to some peculiar unknown structure of the optic nerve, and corresponding region of the brain; viz. the perpetual recurrency of visible objects, either the same or similar ones, during the whole time that we are awake; the distinct manner in which they are impressed by means of the several proper conformations of the eye; and their being received in general upon the same part of the retina, precisely or nearly. 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.
Fourthly, The idea of every familiar object has, for the most part, some particular magnitude, position, and aggregate of associates, in its recurrences to the mind. And this somewhat lessens the difficulty mentioned in the last paragraph. The reason of this fourth observation is, that though every visible object appears under different magnitudes, in different positions, and with different associates, yet these differences destroy one another, so that the strongest particularity only remains. However, changes are made from time to time, each subsisting for a short period, and then giving way to the next in succession.
Fifthly, We have fictitious visible ideas of places and persons that we have never seen, as well as of those which we have. These are derived from association evidently, and they often undergo successive changes, like those spoken of in the last paragraph.
Sixthly, Our visible ideas are subject to the voluntary power in a high degree, and may be called up by the slightest associated circumstance, at the same time that they have very numerous connexions with other ideas, and with actual impressions. The name, or its idea in the region of the brain corresponding to the ear, are the circumstances most commonly made use of for calling up visible ideas. But there are many ideas, i.e. internal feelings, which have no names, and which yet, by attending our several visible ideas, get this power of introducing them.
Here it is to be observed, that an idea cannot be said to be voluntarily introduced, till it be previously determined by some of its associates. If I desire to introduce a visible idea of any kind, an individuum vagum, and that of a horse offers itself, it was not owing to the command of my will, that it was a horse, and nothing else, but to the connexion which the idea of a horse had with some other idea or impression, which then happened to take place. But if I desire to recollect the features of a person’s face, whom I saw yesterday, I make use of his name, his dress, the place in which I saw him, or some other associated circumstance, for this purpose. And this may be called a voluntary introduction of an idea. 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. My design here is, only to suggest to the reader the processes generally made use of in these things. It is to be observed farther, that the associated circumstance, which determines what idea shall be called up voluntarily, does, for the most part, raise it. Thus, if a person desire me to call up the idea of a horse, the very sound of the word proceeding from his mouth will do it, for the most part, immediately. If not, I go back, by my memory, to the trace left by the word, and thence to the idea, or to some common associate of both the word and idea, capable of raising the last.
Seventhly, When we have conversed much with the same visible objects, as after having been in a crowd, travelling, &c. for many hours without intermission, we may find the ideas of these objects peculiarly strong, so as to intrude upon our fancies, and interfere with all our other ideas. This may serve to shew, that the permanence of the sensations impressed, mentioned in the third proposition, and which shews itself particularly in visible impressions, as there remarked, is of the nature of an idea. And it coincides remarkably with this, that the ideas should be peculiarly vivid and precise in the same sense, where the permanency of the sensation impressed is most conspicuous.
Eighthly, The ideas of sight and hearing, and the impressions from whence they proceed, have a peculiar connexion with each other. For as words pronounced call up visible ideas, so visible ideas and objects call up the ideas of words, and the actions by which they are pronounced.
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. This agrees remarkably with what has been already observed concerning the ideas of this sense; and we may infer from all together, that the regions of the brain corresponding to the optic nerve are comparatively large, or peculiarly susceptive of impressions, or both.
Tenthly, The imagery of the eye sympathizes also remarkably with the affections of the stomach. Thus the grateful impressions of opium upon the stomach raise up the ideas of gay colours, and transporting scenes, in the eye; and spasms, and indigestions, have often a contrary effect. 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; because we criticize much upon the beauty of faces, and upon the proportion of the several features to each other; and because evil spirits (the notions of which generally take strong and early possession of our fancies) are painted with ghastly faces. This mixture of reasons hinders each particular one from being so obvious, as might otherwise be expected; however, the same thing is common in many other cases. 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. Our memories are also much assisted by our visible ideas in respect of past facts, and the preservation of the order of time depends in a particular manner upon our visible trains suggesting each other in due succession. Hence eye-witnesses generally relate in order of time, without any express design of doing so. This recollection of visible ideas, in the order in which they were impressed, gives rise to the loci memoriales, in which matters principally worthy of remembrance are to be reposited, and to the artificial memory, that is borrowed from the eye; just as the facility of remembering words formed into verses does to the artificial memory borrowed from the ear. It may deserve notice here, that some persons have imaginary places for the natural numbers, as far as 100, or farther.
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.
Thirteenthly, Painters, statuaries, anatomists, architects, &c. see at once what is intended by a picture, draught, &c. from the perfection of their visible ideas; and carry off the scene, plan, &c. in their memories, with quickness and facility. All which is still owing to association. But it would be endless to enumerate the instances of associations which this sense affords.
Fourteenthly, It is probable, that fables, parables, similes, allegory, &c. please, strike, and instruct, chiefly on account of the visible imagery which they raise up in the fancy. They are also much more easily remembered on the same account. We may add, that idolatry, heathenish and popish, has made a much quicker and more extensive progress in the world on account of the stability and vividness of visible impressions and ideas, and the difficulty, obscurity, and changeable nature, of abstract notions. And image worship seems even to have been derived in great measure from this source.
Fifteenthly, It would be a matter of great curiosity and use (as far as these speculations can be of any use) to inquire carefully into the progress of the mind, and particularly the fancy, in persons born blind, and compare the result with what is advanced under this proposition, and with other parts of these papers, in order to correct and improve the theory of association thereby. It is probable, that they are considerable losers, upon the whole, in respect of knowledge; though their greater degree of attention, and the superior acuteness of the senses of feeling and hearing, and, consequently, perfection of the ideas of these senses, must give them some particular advantages.
These motions are of two kinds, external and internal. The external are the motions of the globe of the eye, those of the eyelids, and the contractions of the lacrymal, and other neighbouring glands, whereby they are evacuated. The internal are the contractions of the greater and lesser muscular rings of the iris, of the radiated fibres of the iris, and of the ciliar ligaments. I will speak of each of these in order.
I begin with the motions of the globe of the eye. And here I observe, first, that the white tendinous expansions of the four straight muscles reach as far as the cornea; and consequently, that they are thereby exposed in a particular manner to the action of light, when the eye is open, being covered with nothing but a thin membrane. However, the tendinous expansions of the adducens and abducens are much more exposed than those of the attollens and deprimens; and if the eye be but a little open, the light cannot fall upon these last at all.
Secondly, If a luminous object be placed upon the right side, so as that the light shall fall in a particular manner upon the tendinous expansion of the abducens of the right eye, and of the adducens of the left, it may by contracting these muscles make the eyes move in a congruous manner, turning them towards itself; and the tendency of the eyes to move towards the right side ought not to cease, till the adducens and abducens in each eye have an equal quantity of light fall upon them from the luminous object, i.e. till the optic axis be directed to it. In which case the eyes would be in æquilibrio, as far as they are under the influence of the light which falls upon the tendinous expansions of the straight muscles.
Thirdly, It agrees remarkably with the two last paragraphs, that new-born children move their eyes in a congruous manner; that the motions are chiefly to the right and left, scarce upwards and downwards at all, the eyelids being seldom so much opened as to expose the tendinous expansions of the attollens and deprimens; and that their eyes are frequently turned towards luminous objects, such as a candle, or a window.
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. 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.
Fifthly, It is worthy of attention here, that the attollentes and deprimentes do not want the same external influence of light to make them move in a congruous manner, as the adducentes and abducentes; inasmuch as one adducens, and one abducens, must act together to make the eyes move congruously to the right and left; whereas the two attollentes, and two deprimentes, act together in the congruous motions upwards and downwards. As far therefore as the nerves of one side sympathize with the corresponding nerves of the other in the influences which descend from the brain, there will be a natural tendency in the eyes to move upwards and downwards in a congruous manner, and to the right and left in an incongruous one. And this suits well with the greater exposition of the tendinous expansions of the adducentes and abducentes, before taken notice of. For what reason so many pairs of nerves are concerned in the motions of the globe of the eye, and of the eyelid, remains to be inquired.
Sixthly, It may perhaps be, that the light which passes in at the pupil has some efficacy in moving the globe of the eye, either by unknown communications in the brain between the optic nerve, and the third, fourth, and sixth pairs, or perhaps by penetrating in a small degree through the retina, choroides, and sclerotica, to the four straight muscles. If this last influence could be allowed, it would oppose that exerted upon the tendinous expansions; but would, however, join with it in preserving the congruity of the motions.
Seventhly, As the two oblique muscles neither have tendinous expansions exposed to the light, nor adhere to the globe of the eye, except just at their insertion, they cannot be under either of the influences here supposed to affect the straight muscles, but must be subject chiefly to those which descend from the brain; thus acting almost uniformly, unless in particular agitations of the whole nervous system. And this agrees well with the fact, and with the uses generally assigned to these muscles, viz. those of keeping the eye in a moderate suspension always, and drawing it out on eminent occasions.
Eighthly, The circumstances which occasion squinting in young children, agree well with the theory here proposed. 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. What turns the scale in favour of this position, remains to be inquired.
The second of the external motions is that of the eyelids, or the actions of the elevator, and orbicularis palpebrarum. 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. As to the orbicularis, it is evidently put into action by irritations affecting the eye, as from dust, flies, &c. even in adults.
The third of the external motions, or the contraction of the lacrymal and other neighbouring glands, arises from irritations in the eye, nose, fauces, and scalp of the head. The manner in which these irritations operate, has been sufficiently explained under Prop. LIII.
Of the internal motions I will consider the contraction of the greater and lesser rings together, as proceeding from the same causes, and being cotemporaneous; and, for the same reasons, the contraction of the radiated fibres, and ciliar ligament, together also. By the first the eye is fitted for distinct vision at small distances, by the other at great ones. Thus, let us suppose a candle to be brought nearer and nearer to the child’s eye. It is evident, that the quantity of light which falls upon the eye will grow greater and greater. It will therefore agitate all the circular fibres of the iris more powerfully, and particularly the greater and lesser rings; i.e. it will bend the cornea into a greater convexity, bring the origin of the ciliar ligament nearer to its insertion in the capsula of the crystalline, i.e. suffer the capsula to become more convex also, and narrow the pupil, i.e. lessen the radius of dissipation. The image of the candle upon the retina may therefore continue to be distinct, as it approaches, by this mechanical influence of light upon the eye. And, for the same reasons, it may continue distinct, as it recedes. But there are limits on both hands. And thus the conformations of the eye necessary for distinct vision, according to Dr. Jurin’s most accurate account of this matter, are brought about automatically, and suitably to the general theory of these papers.
However, it is also probable, that the light which passes in at the pupil, has great efficacy in contracting both the greater and lesser rings, as may be concluded from the immobility of the pupil in a gutta serena; also because, on this supposition, the light, which passes in at the pupil must, by contracting the lesser ring, become a check and guard against its own too free admission, which is agreeable to the tenor of nature in like instances. The retina extends to the greater ring, and may send some nervous fibres to it, and even to the iris.
One or both of these actions of light seem to increase the secretion and circulation of the aqueous humour in new-born children, so as to fit the eye for vision, which it is not, through the deficiency and muddiness of the aqueous humour, till some time after birth. This again may be considered as a circumstance that favours our present conjectures.
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.
Since the motions of the eyes are in every instance congruous, from the instant of birth, an associated tendency thereto is generated sooner, and more firmly established, than perhaps in any other case. 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.
Now, here we are to observe, that the almost perfect congruity begets an almost perfect correspondency in the points of the retina; and that hence it will follow by degrees, that the least deviation from perfect congruity will occasion double vision, and confusion. But these are unpleasant and inconvenient, whereas single and distinct vision is pleasant and convenient. Whence every recurrency of the last will tend to confirm it, of the first to exclude it, from principles already laid down, Prop. XXII. Cor. I. The child will therefore come to a perfect congruity at last, i.e. to direct his optic axes precisely to the point which he intends to view. The voluntary power of suiting the internal motions to the distance must be supposed to grow perfect about the same time.
It agrees with this method of reasoning, that persons who lose the sight of one eye in their childhood or youth, though long after the external action of light has lost its efficacy for making the motions congruous, generally squint a little with that eye.
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.
It is generally some time before children get the voluntary power of shutting the eyes gently, or of shutting one and not the other. They can shut them with force soon, this action recurring often from motes in the eyes, pain of any kind, &c. The procedure here is of the same kind as in other voluntary actions.
The internal motions depend originally upon the greater or less quantity of light which falls upon the cornea and iris, as has been observed already. But the nearness and remoteness of the luminous object are the respective associates of these. The muscular rings therefore, which at first contracted only when very luminous objects approached, will afterwards contract when moderately luminous ones do. And thus vision will be made distinct in general. But distinct vision, by recurring, will perpetuate and perfect itself, and indistinct check and abolish itself, from the agreeableness and disagreeableness accompanying them respectively, by Prop. XXII. Cor. I. till at last the child gets a perfectly voluntary power of suiting his eyes to the distance. 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.