Leonardo da Vinci - the anatomist (1930) 18
Embryology - 24 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
Leonardo da Vinci - The Anatomist
Chapter XVIII The Sense Organ
When it was the structure of a sense organ that was in question, and it was the structure of the eye alone that was considered in detail, the Arabian and mediaeval writers w r ere content to rely on Galen ; but when they considered the functions of these organs and how a sensation entered into consciousness they went back to Aristotle for the foundations of their theories. Consequently, to understand Leonardo’s attitude toward the physiological and psychological problems presented by the sense organs it will be well to consider briefly Aristotle’s psychological ideas.
Every object, he claimed, may be regarded as a combination of matter and form, this latter term being used in a very broad sense, so that it might mean the sweetness of a sapid substance, or color, or hardness, as the case might be. Sensation is the perception of objects in their forms, independent of their matter, and is the result of a change set up in the sense organ by the object, either directly or indirectly. Each sense organ reacts to only one variety of form ; but an object may have several forms; in addition to color it may have, for example, number, size, motion, and for the perception of all these and for the coordination of sensations perceived by various sense-organs a sensus communis is predicated. By this coordination of sensations a mental representation of the object perceived, a “phantasy,” is formed and such representations form the basis for thought and memory.
Aristotle’s anatomy was discredited because he located the common sensorium in the heart, the source and center of the pneuma, believing the brain to be wholly destitute of sensation, and being misled by his failure to distinguish between nerves and tendons. In these views he stood alone, for older writers, and even his predecessor and teacher Plato, located sensation in the brain. The Alexandrian anatomists determined the origin of the nerves from the brain and spinal cord and later Galen’s authority led to the general acceptance of the brain as the location of the sensus communis and the other mental faculties, whose supposed distribution in the brain has already been considered.
Only the five classical senses were recognized by Aristotle and, indeed, were believed to be the only ones possible; since they give information of all objects known to us, any additional ones could only be duplications and, therefore, superfluous, a condition impossible in his scheme of the cosmos. He found difficulty, however, in regarding touch as a single sense, since the sensation of temperature was associated with it, and he endeavored to avoid the difficulty by supposing that touch gave information as to the actual object, but associated with this were two pairs of contrasting qualities, fluidity and solidity, heat and cold, the qualities of the four elements that entered into the composition of the object. Avicenna added to the difficulty by recognizing in touch four pairs of discriminations, heat and cold, dryness and wetness, hardness and softness, roughness and smoothness, still adhering, however, to the doctrine of five senses, and in this he was followed by Leonardo, who does not commit himself as to the composite character of touch.
According to Aristotle’s opinion, that sense was the most perfect which could most correctly perceive the form of the object as apart from its matter. Touch was, therefore, the least perfect sensation, since it required actual contact with the material object. Taste was somewhat more perfect, since, though it required contact with the sapid body, it gave more than the sensation of mere contact. Smell came next, then hearing and finally r sight, none of these three requiring actual contact, but acting through a medium interposed between the object and the sense organ. This order may be followed in considering Leonardo’s treatment of the sense organs, though it may be premised that it is sight alone that he considers at length.
Of touch he notes that it is distributed over the whole surface of the body (AnB, 2), instead of being located in a definite organ as are the other senses, and the impression passes 1 to the sensus communis by the perforated cords, i.e. the nerves, which are spread out with infinite ramifications in the skin enclosing the members of the body and the viscera. It has been shown (p. 208) that Leonardo was bold enough to modify the time-honored ideas as to cerebral localization by transferring the sensus communis from the anterior to the middle ventricle, and he also suggests (QV, 7) that since the posterior ventricle is in the region where all the nerves that give the sense of touch concur, that is to say the spinal nerves through the spinal cord, it may be that the scat of the sense of touch is in this posterior ventricle, “since Nature in all things operates in the briefest time and way possible.” He notes, perhaps from observation of the results of a wound, that if the nerve to the finger is cut the sensation of the member is destroyed “even if it be placed in the fire” and states that in order that such nerves may be protected from injury they are placed between two fingers (AnA, lOv). One finds in two passages (AnA, 10 and 13v) an account of the illusion described long before by Aristotle, to the effect that if two fingers of one hand are crossed one object placed between them will seem, so far as the tactile sense is concerned, to be two. He fails, however, to understand that the illusion is a mistake in judgment, suggesting that it is due to the fact that the two surfaces of the fingers concerned are supplied by different nerves, whereas when the object touches the adjacent surfaces of uncrossed fingers both of these surfaces are supplied by the same nerve; by nerve, here, he means of course nerve branches.
- The text reads ‘‘does not pass,” but the non is evidently an error.
Of temperature sense Leonardo makes no mention, probably including it in touch, nor does he have anything to say as to taste. Smell he locates in the sensus communis (AnB, 2) and evidently held that it was due to the contact with the olfactory organ of particles given off from the odorous substance and transmitted through the air as a medium. He speaks of musk as always keeping a great quantity of the atmosphere charged with its substance, without, however, diminishing in quantity (CA, 270).
The structure of the ear was very imperfectly known to the classical writers, even to Galen; it was for them a cavity of more or less complicated form, excavated in the substance of the petrous portion of the temporal bone. This too was Leonardo’s idea, who states that to hear the sound of a voice it is necessary that it should “resound in the concave porosity of the petrous bone, which is to the inner side of the ear,” whence it is carried to the sensus communis (AnB, 2). A sound spreads itself out in circles through the air (A, 9v), without, however, producing displacement of the air (CA, 360), and, just as light is reflected from a mirror with the angles of incidence and reflexion equal, so will the note of an echo “strike and rebound within the hollow where it has first struck with equal angles to the ear” (C, 16). The comparison with vision suggests the query whether there may not be an aftersound in the ear, just as there is an after-image in the case of sight; whether the sound that remains or seems to remain in the bell, after it has received the stroke may be not in the bell but in the ear, but this idea is rejected since “if it were true, the sound of the bell would not cease abruptly if the bell is touched by the hand — but it does” (CA, 332).
Leonardo’s interest in the eye was principally and primarily that of the artist, seeking the principles that determine differences in the appearance of objects due to differences in their illumination, or those that determine the apparent size and form of objects at various distances, in order that he might more accurately portray them. But, as in other cases, he was quickly led beyond the view-point of the artist into the investigation of the problems of physiological optics, into inquiries as to the mechanism by which images of objects were conveyed to the sensorium. Unfortunately in pursuing these studies he neglected to obtain an accurate knowledge of the machine itself, but was content to rely upon the erroneous ideas as to the structure of the eye that had been handed down by earlier writers, with the result that he contributed little if anything to the knowledge of physiological optics.
The coats of the eyeball and its contents were well described by Galen and his description was satisfactorily recapitulated by Avicenna.
The membranes of the brain were supposed to form a sheath for the optic nerve and at its termination expanded to form the coats of the eyeball, enclosing its contents, the humors. In the center was the lens or crystalline humor, termed by Avicenna, likening it to a hailstone, the glacialis. Posteriorly is the vitreous humor, like liquefied glass and embracing the glacialis as far forward as its equator, and anteriorly is the third or aqueous humor, termed by Avicenna the albugineus, likening it to white of egg. At its termination in the eyeball the optic nerve expands into a membrane, net-like in texture, whence it was termed the retina, and this extends forward to the boundary between the albugineus and glacialis, where it forms a membrane, the tela aranea, that separates these two humors. External to the retina is the secundina, now known as the chorioid coat, which is interwoven with veins for the nutrition of the eyeball and anteriorly becomes converted into a membrane of a blue color, the iris, which is perforated by a foramen, the pupil. External to the secundina is a dense membrane, whose posterior opaque portion is termed the tunica dura, while anteriorly it forms the transparent cornea. Finally there is the most external coat, the conjunctiva.
From this description, read as a guide to the dissection of a bullock’s eye, a fair idea of the structure of the eyeball might be obtained. But mediaeval anatomists did not dissect, and the mental picture they obtained from Avicenna’s description was very incorrect, as is evidenced by the figures illustrating their treatises. In two of Leonardo’s figures of the eye (QV, 6v) the conjunctiva is not represented, the lens is a spherical structure occupying the center of the eyeball, the sclerotic and choroid coats are incomplete anteriorly, a pore traverses the tela aranea and the aqueous and vitreous humors are not identified. In a third figure (CA, 1128) (fig. 82) somewhat more detail is shown, but the large spherical lens still occupies a central position. Notwithstanding Leonardo’s suggestion of a method for preserving the relations of its parts during dissection (p. 87), his figures give no evidence that he had made use of the method. They rather give the impression of being copies from memory of an earlier figure, but what that may have been is uncertain; they bear some resemblance to a figure in the Margarita Philosophica of Gregor Reisch (1508) and may trace back to a common source with it. The supposed central position of the lens was a bar to the correct understanding of the dioptrics of the eye and, it may be noted, it continued to be so until long after Leonardo’s time. Vesalius in his De fabrica (1543) assigned to the lens a central position and it was not until 1583 that Felix Plater recognized its true position.
Leonardo’s uncertainties as to the structure of the eye are perhaps the cause of the uncertainties of his nomenclature, all the more striking in comparison with the definiteness of that of Avicenna. The cornea is generally termed luce, but occasionally that word evidently means the pupil (D, 5; AnB, 13v), elsewhere termed popilla; the lens is usually the spera crystallina, but on D, 8 it is termed the spera vitrea; the albugineous humor has a spherical form and the lens is situated at its center, no distinction being made between the albugineous and vitreous humors (D, 3v) ; and, finally, the term uvea seems to denote the retina (D, 3v, 7v, 10).
Fig. 82. Diagram of the structure of the eye. (CA, 337 II., A.)
Fig. 83. Diagram showing two possibilities of refraction within the eye. (D, 10.)
To the ancient Greeks it seemed that sensation could occur only by the contact, direct or indirect, of the object perceived with the sense organ. With most of the senses the contact seemed obvious, but it was not so with vision. Two theories of sight contact suggested themselves. One, adopted by the Epicureans, assumed an extension of the object to the sense organ, minute particles of the object being given off to the air and being carried by it to bombard the sense organ; the other, adopted by the Stoics, assumed an extension of the sense organ to the object, the pneuma or visual virtue being projected from the eye so as to embrace or include the object seen. Plato, however, dissatisfied by both theories, suggested a modified combination of them. He assumed that visual virtue emanated from the eye, and, meeting and combining with emanations from the object, returned to the eye to produce in it an image of the object. The air acted as a medium for the transmission of the emanations, but it was air modified by these that established the contact.
Later authors, such as Galen and Euclid, accepted Plato’s theory, but still later it was replaced by that formulated by Aristotle. His theory of vision was primarily an attempted explanation of color vision, but granting that it might account for this, it might also suffice for the explanation of form. It was obvious that the eye and the object seen were both essential to vision — either being absent it could not occur; but the third factor, the medium, was quite as essential and if it were eliminated, as by placing the object directly upon the eye, again vision failed. The eye and the object were both concrete and definite, the medium offered a greater field for assumptions, and it was upon the medium that Aristotle concentrated his attention. It was as a rule air, but it might be other material, water for example, and therefore the true medium must be something possessed by both air and water, that is to say translucency, a property due to the presence in each substance of a constituent which Aristotle termed the diaphanous. It was upon this diaphanous, an adumbration, if one may use that word, of the luminiferous ether of more recent times, that the object impressed its color and, less readily, its form, so that it carried simulacra or, as they came to be termed, species of the object, and vision resulted if these encountered the eye. This theory made emanations from the eye superfluous and it also obviated the difficulty of explaining how emanations from the object could penetrate the eye, since the cornea, the lens and the two humors were translucent, that is to say contained the diaphanous, and so the species could be carried to the optic nerve and by it to the sensorium commune.
It may be presumed that Leonardo was more or less familiar with Aristotle’s theory of vision, for while he does not speak of the diaphanous medium he regards vision as due to the entrance into the eye of simulacra (species) of the objects seen.
“The senses when they receive the simulacra of things do not send forth from themselves any actual power, but the air between the object and the sense incorporates in itself the simulacra of things and by contact with the sense presents them to it.” (CA, 80.)
Every object fills the surrounding air with its similitudes and the air is accordingly filled with infinite straight and radiating lines, mutually intercrossing and interlaced, but without interference (A, 2v), and it is along these lines that the species travel from the object to the eye (A, 8v), taking the shortest and most direct course in accordance with the law propounded by Aristotle that every natural action is performed in the shortest way possible (D, lOv). When one considers the wide range of one’s vision and the number of objects that may be perceived at a glance, one may form a feeble conception of the innumerable lines, each bearing its own species, that must be co verging upon him. But this is but a small part of the story, for as many other lines must be converging upon every individual that perceives the same objects from slightly different angles.
“The image of the sun is single in all the sphere of water that sees it and is seen by the sun, but it appears to be divided into as many parts as there are eyes of animals seeing it (reflected) from the water. Sailors see the sun at the same time in all parts of their hemisphere.” (D, 6.)
The magnitude of this concept is stupendous and it startled even Leonardo, bold as he was in conjecture, that in the eye the image of the moon in the east and that of the sun in the west might be brought together in a single’ picture.
“Who” he says “would believe that so small a space could contain the images of all the universe? 0 mighty process! What skill can avail to penetrate a nature such as thine! What tongue can unfold so great a wonder! Truly, none! This it is that leads human discourse to the consideration of things divine.” (CA, 345v.)
Sometimes, indeed, he doubts ’whether this rectilinear emission from the object is the true explanation of vision, for, in speaking of the rays of luminous bodies, he says —
“It is necessary to determine first what these rays are and to see whether they have their origin from the eye that sees the object or from the luminous body.” (D, 9v.)
This was apparently, however, but a passing suggestion ; he held it to be impossible that the eye should project from itself the visual virtue (Ash 2 , 1); nevertheless he maintained that subtle influences, unconnected with vision, emanated from the eyes. He disputes the opinion of those who held that such emanations would lessen the power of vision and would produce a diminution of the eye, citing in analogy the case of musk, which perfumes great quantities of air without itself being diminished, and that of a bell, which gives forth sound when struck and yet is not consumed thereby. And he then proceeds to give concrete examples of such emanations, such as the serpent luring by its fixed gaze the nightingale, the lethal glance of the basilisk, the glance of the wolf which produces a hoarse voice, the gaze of the ostrich and spider which is said to hatch their eggs, the beguiling power of maiden’s eyes and finally the case of a Sardinian fish that is said to emit light from its eyes which kills other fish (CA, 270v). Mediaeval credulity, with its faith in Pliny’s tales, was still strong in Leonardo’s time.
These emanations were, however, something quite different from the spetie emitted by visible objects and passing in straight lines to the eye, where they are received by the luce or cornea. What then was supposed to be their fate within the eye? Where was seated the virtu visiva by which the “species were combined to form an image of the object from which they came?” It must be remembered that in Leonardo’s time the function of the retina was unsuspected; he seems to have regarded it as a reflecting surface, comparing it to a silvered mirror (D, 7v), an idea suggested, perhaps, by the observation that if one stand between a cat and a light the animal’s eye appears to be on fire (H, 109). The retina then being hors concours, what were the other possibilities?
In some passages Leonardo speaks as if he regarded the pupil as the seat of the visual virtue; he states, for example, that the pupil has all the virtu visiva in all parts and all in each part (D, 6v; AnB, 25). But this was either merely a passing idea or a careless method of expression, since, elsewhere, he notes that —
“The pupil is situated opposite the middle of the cornea, which has the form of a portion of a sphere. It receives the similitudes of objects and sends them by the pupil within, to the place where vision is formed.” (K, 119.)
Within the pupil is the lens and this too is considered.
“Species that penetrate the albugineous humor by way of the pupil meet in the sphere of the crystalline humor, and either the virtu visiva is in it or at the extremity of the optic nerve, which receives the species and refers them to the common sensorium. ... If the virtu visiva is in the center of the crystalline lens, this either receives the species on its surface, they being sent to it from the surface of the cornea where the objects see themselves, or they are reflected to it from the surface of the uvea (retina) which surrounds and covers the albugineous humor.” (D 7v.)
This passage presents as an alternative for the lens the end of the optic nerve, and without further quotation it may be said that Leonardo finally decides upon this (D, 7v, 8; AnB, 2). There is a flavor of irony in the fact that he should have located the virtu visiva in what is now known to be the blind spot of the eye !
The decision in favor of the end of the optic nerve was based on an attempt to explain the erection of the image. The phenomena of refraction had been quite accurately described in the eleventh century by the Arabian mathematician Muhammed Ben el Hasan Ibn el Heithem, better known to the western world as Alhazen, and were known to Leonardo and his contemporaries. Leonardo recognized that the cornea and lens must produce refraction, that there must be indeed a crossing of the rays coming from an object and therefore an inversion of the image. How was it then that it appears erected? It does not seem to have occurred to him that the image is received by the virtu visiva inverted and that its erection is a matter of interpretation or experience and, accordingly, a function of the sensorium commune or other part of the brain. He was therefore forced to the conclusion that there was a second crossing of the rays within the eyeball.
Figure 83, a reproduction of a diagram on D, 10, illustrates two ways in which this double crossing might occur, one of the ways being later condemned. The species from a, b, c converge as they approach the eye and pass through a nodal point in the aqueous humor, so coming to the anterior surface of the lens, upon which the image would be inverted. The species may then follow one or other of two courses; either they may be refracted by the lens and so cross a second time, producing an erected image at the end of the optic nerve, prolonged in the diagram so that it is in contact with the lens, or they may be reflected from the surface of the lens to the retina at p and q, whence they are again reflected to the end of the optic nerve. This second path, would, however, leave the image inverted and it is therefore discarded.
“The pupil always receives the images of objects upside down and the virtu visiva sees them as they are. This is due to the rays passing through the center of the crystalline sphere in the middle of the eye.” (D, 2v.)
It is noted that the double crossing in the eye might be open to objection and the erection of the image ascribed to the decussation of the optic nerve (D, 3v).
How far Leonardo 'was from a correct understanding of the dioptrics of the eye is evident from the figure. But even so he was in the line of progress in locating the seat of vision in the nerve, other structures in the eye being merely contributory to effective stimulation. For Galen had transmitted the theory that the lens was the seat of the visual virtue, basing his argument mainly on the idea that vision was a sense sui generis, and that since there was nothing like the lens elsewhere in the body it must be the seat of vision.
Leonardo experimented extensively with vision through a pinhole, finding in these experiments his strongest evidence for the primary inversion of the image. He figures (D, 10) a pinhole opening in the wall of a rectangular box and notes that the image thrown upon the opposite wall must be inverted. He was working with a pinhole camera obscura and evidently recognized that the eye might be regarded as such a contrivance. Indeed, the invention of the camera obscura has been ascribed to him. One pinhole experiment interested him greatly. Looking through a pinhole at a source of light and holding a pin between the perforated card and the eye, one sees the image of the pin inverted and apparently beyond the card, and if the pin is moved upward its image moves downward (D, 4v). Leonardo’s argument from this observation may be supposed to be as follows. Since the camera obscura shows that there is a crossing of the rays at the pinhole and since the image of a pin beyond the pinhole appears erect, there must be a second crossing in the eye. If then one eliminates the crossing at the pinhole by holding the pin between the eye and the card, only one crossing affects the image and it should appear inverted, and so it does. Unfortunately for his argument, Leonardo failed to perceive that with the pin between the eye and the pinhole it is the shadow of the pin, interrupting rays that have already crossed, and not the pin itself that is seen, and since the shadow on the retina is that of an erect pin, it is interpreted as being inverted The same result may be obtained by bringing the perforated card so close to the eye that the shadows of the eyelashes are seen, when if the card be moved to the right, the shadows seem to move to the left (K, 125 v).
How a visual estimate of the size of an object is made is a question that would naturally attract an artist, and Leonardo devoted no little thought to it, without, however, reaching any very satisfactory conclusions, since he knew little of the mechanism of accommodation. He laid down two principles governing the perception of size, (1) that that object appears largest whose species come to the eye at the greatest angle (E, 15v) and (2) that of two objects of equal size that which is the more brilliantly illuminated will seem the larger and the nearer (C, 1) In connection with the first of these principles there seems to have arisen in Leonardo’s mind some confusion of the angle of vision with the angle of incidence of the visual rays, for he states in one passage that “the eye has in it an indivisible point to which the apices of all the pyramids ( i.e . cones of visual rays) converge from different objects,” this point being at the center of the eye, that is to say in the lens (A, 10). But if this point is the visual center then there is an obvious difficulty.
“If all images came at an angle they would concur in a point and all things in the Universe would appear one. We conclude, therefore, that the sense takes the images that are reflected at the surface of the eye, and then judges them within. They do not concur in a point, nor in consequence in an angle.” (F, 34.)
The distinctness of an object was held to depend not only upon the brightness of its illumination, but also on the size of the pupil.
“The greater the size of the pupil the more of form and brightness will it see in the object and, conversely, the smaller it is the less and more obscurely will it see its object.” (AnB, 25.)
This leads to observations on the dilation and contraction of the pupil.
“If the eye that has been in the dark is suddenly illumined, the pupil contracts so that the light will not injure too great a portion of it. In darkness the pupil dilates to its full extent to send the resemblance of obscure things to the imprensiva.” (C, 16.)
It is noted that the power of adapting the pupil to the amount of light is especially marked in nocturnal animals.
“Nocturnal animals, like cats and owls, have a small pupil and at night a very large one. So with all terrestrial animals and those of the air and water, but more beyond compare, the nocturnal animals.” (D, ov.)
“In all animals the pupil dilates or contracts according to the amount of light. It is greatest in birds, especially in nocturnal ones, such as the owl. In this the pupil may occupy the whole eye or contract to the size of a grain of millet, remaining circular. In the lion species the pupil contracts from a circular to a triangular (i.e. slit-like) form. Man, less blessed with vision, is not harmed by excess of light. The eye of the owl increases so much that it sees better in the smallest light of night than we do in the full splendour of mid-day.” (G, 44.)
“The luce or pupil of the human eye in its dilation and contraction, dilates or contracts by half its size, and in nocturnal animals it contracts and dilates more than a hundredth part of its size. And this may be seen in the eye of the owl, a nocturnal bird, by bringing toward its eye a lighted torch, or better if you make it look at the sun, for then you will see the pupil, which at first occupied all the eye, contract to the size of a grain of millet, and in this contraction it equals the pupil of man and clear and lustrous objects appear of the same color as, in such time, they appear to man, and so much the more that the brain of such an animal is less than the brain of man. Whence it happens that such a pupil increasing in the night-time a hundred times more than that of man, sees a hundred times more light than man, so that the power of vision is not then overcome by the darkness of night. And the pupil of man, which only doubles its size, sees little light and is almost like that of a bat, which does not fly in times of too much darkness.” (AnB, 13v.)
“Only in the lion tribe among animals does the pupil alter its shape as it grows larger or smaller. In full contraction it is long, in moderate oval and at full expansion circular.” (C.A., 262.)
“The ventricle of the brain termed imprensiva is more than ten times the size of the eye of man, of which the pupil (luce), which gives the vision, is the thousandth part. In the owl the nocturnal pupil is as large as the imprensiva. In man the imprensiva is like a large hall lighted by a small hole, but in the owl it is like a small chamber altogether open. For in the large hall it is night at mid-day and in the small open one it is day at midnight, when the weather is not cloudy.” (D, 5.)
These passages tell their own story. The effect of light on the pupil was clearly perceived, but Leonardo failed to observe the movements of the iris associated with accommodation, first described by Scheiner in 1631. In one passage he speaks of changes in the lens—
“The crystalline humor which is internal to the pupil condenses to meet the things that shine and rarefies to meet obscure things.” (D, 5v.)
But this supposed change was not associated with the idea of accommodation for distance, a phenomenon of wdiich he was ignorant, the part played in it by the lens having been first suggested by Descartes (1637). Nor did he note the value of the iris in diminishing spherical aberration, for this too was unknown to him. He did recognize, however, that the species that reached the eye in the line of the axis of vision were more distinctly perceived than others. Thus he says —
“An object is less distinct if it impresses itself at a greater distance from the center of the pupil, where terminates the median line that goes straight to all the objects of the figure of which one can have a true and certain knowledge. This line is straight, without any intersections, and is the mistress of other lines.” (D, 8v.)
And again —
“The eye has in it a single line placed in the middle of an infinity of others; to which it is the central line, and these others have more or less power as they are nearer or further from this central one.” (D, 8v.)
He notes that if there is a number of objects in a line extending away from the observer and vision is concentrated on one, the others will appear double. Thus if there are five objects, the number seen will be nine, if two objects three, and if one hundred, one hundred and ninety-nine. So too he notes that an opaque object, smaller in diameter than the pupil, placed before the eye, does not prevent vision of a more distant object, but behaves as if it were transparent (D 6v), a statement that needs some qualification.
Again it is noted that a luminous object, such as a star seen through a pinhole, loses its rays and appears round and smaller — smaller, because with the pinhole a smaller amount of the pupil is affected, and the size of the image is proportioned by the amount of the pupil affected by its rays (F, 32) ; round, for a remarkable reason,
“The rays which show at the extremities of luminous bodies have their origin not in the bodies, but in their images impressed in the thickness of the eyelids. In looking at a star each eyelid receives an image and the pupil another, and the three images, being adjacent, appear one to the eye. If we raise the head the eve will lose all the lower ravs of the luminous body.” (D, 9v.)
And essentially the same idea is expressed in F, 30 and D, lv, with the additional statement that the images received through the eyelids wall be inverted, since the fluid that lubricates the eyelids wall be concave “according to the fourth and the eighth of the Waters,” and the pupil is within the concourse of the pyramidal rays of the concave mirror. Confusion also seems to prevail where the perception of the boundaries of objects is considered (D, lOv; E, 15). The eye is never able to perceive boundaries clearly, apparently because they wall be confused with “the images of more distant bodies, but the nearer the object is to the pupil, the more distinct will be its boundaries.”
The imperfections of vision due to the shape of the eyeball are not considered, although the use of eye-glasses w T as known, and it was understood that they acted by refracting the rays before they reached the eye (D, 2). In one passage (G, 90) Leonardo speaks of presbyopia, explaining it by assuming that the aged are far-sighted because an object sends less impression of itself into the eye if it is far off than if it is close. Just as changes in the lens to produce accommodation w 7 ere unknowm, so w r ere those due instanti senectce.
The persistence of the visual image w 7 as recognized and was regarded as an illustration of a general tendency, according to w T hich “every impression tends to permanency or desires permanency” (G, 73). Ocular after-images w 7 ere regarded as evidence that the images of objects enter the eye (CA, 204). A glowing brand moved rapidly in a circle gives the impression of a continuous line because the brand passes through infinite adjacent lines (QIII, 12v) along each of w 7 hich an image passes to the eye in such rapid succession that they appear to be continuous. There is room for doubt, how 7 ever, w 7 het.her Leonardo saw in this phenomenon the effect of after-images, nor did he assign to them the doubled appearance of a vibrating string, w T hich he explains by supposing that the string moves so rapidly that the eye gets simultaneously two images of the thing moved (C, 15). In one passage (G, 73) he seems to consider ocular after-images to be of the same nature as the persistence of auditory impressions after a bell has been struck by a hammer (G, 73), but elsewdiere he comes to the conclusion that the continued sound impressions are due to continued vibration of the bell and are not in the ear, whereas after-images are unquestionably in the eye (CA, 332).
As to the question of binocular vision, Leonardo has little to say, but some of that little is very pertinent. Since the brightness or distinctness of any object w r as supposed to be due to the amount of the pupil affected, then it w 7 as natural to assume that an object would be less clear if seen by only one eye (AnB, 25; H91v).
“If one eye be closed and sees darkness and the other is open, the light it sees is mingled with darkness in the virtu visiva.” (D, 9v.)
This smacks of theory rather than observation, but another memorandum shows that he had hit on the idea of stereoscopic vision, later worked out in detail by Wheatstone in 1830. On H, 49 Leonardo notes that objects seen with both eyes will appear rounder than those seen with one eye, and in the Trattato della Pittura he notes that if the right eye is fixed on an object, all other objects in the same line will be hidden from it, but may be seen with the left eye. Thus the two eyes get different views of an object; they see it from slightly different angles.
Finally, as to color vision, or rather the physiological optics of color vision, Leonardo has nothing to say. As an artist he was of course deeply interested in color effects and he notes the effect of contrast on color perception, dark drapery, for instance, making the flesh seem whiter and lighter drapery making it darker. But how perception of color differs from that of form he does not discuss. Indeed it was not a problem in his day; color was included in Aristotle’s category of form as distinct from substance, and the species that passed from the object to the eye were simulacra of color as well as of configuration, and color, consequently, needed no further consideration.
| Historic Disclaimer - information about historic embryology pages |
|---|
|
Reference: McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.
Cite this page: Hill, M.A. (2024, April 24) Embryology Leonardo da Vinci - the anatomist (1930) 18. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Leonardo_da_Vinci_-_the_anatomist_(1930)_18
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
