Leonardo Studies II · Chapter 11 of 18
Part Three, Chapter 2: The Visual Process
1. Introduction 2. Extromission Theory 3. Intromission Theory 4. Point Theory
5. Possibility of Intersection 6. Intersection and Divergence beyond the Point
7. Small Objects in front of Eye 8. Apertures and Contrary Movement of Objects
9. Single or Double Inversion 10. Models 11. Conclusions
1. Introduction
In Antiquity, questions whether vision occurs through rays emanating from the eye, extromission, images coming to the eye, intromission, or some combination of the two had remained a matter for philosophical debate1. Such questions were outside the scope of optics. As Hero of Alexandria explains in his Definitions:
Among the mediaeval Arabic writers on optics this attitude changed. Alhazen, writing in the early eleventh century, advanced demonstrations to show that vision occurs through a passive intromission of images.2 This became the dominant interpretation but alternative theories continued to hold their ground. Albertus Maganus, in his commentary on Aristotle's De sensu, notes no less than four explanations, each of which he ascribes to an ancient author:
a. emanation of visual rays (Empedocles);
b. emanation of visual rays which meet incoming rays (Plato);
c. intromission involving corporeal images (Democritus);
d. intromission involving spiritual images (Aristotle).3
In addition Albertus Magnus alludes to, but rejects, a "somewhat new" theory claiming that vision involves a simultaneous extromission and intromission of rays. In the fifteenth and sixteenth centuries these competing explanations were perpetuated in the many commentated editions of Aristotle.
At the same time Renaissance thinkers transformed their mediaeval sources. Alhazen had been one of the staunchest supporters of an intromission theory of vision. His sixteenth century editor, Risner, inserted a chapter (I:24) entitled: "Vision appears to be made by uvauy av, that is, rays that are simultaneously received and emitted." Through Risner, Alhazen could be seen to support a view he himself would have rejected.
Even the landmark demonstrations of Kepler (1604) and Scheiner (1619) did not settle conclusively this debate concerning the visual process. In the mid-seventeenth century thre are still numerous debates in the universities on the question whether vision occurs through extromission or intromission.4 This context helps us to understand why Leonardo should consider alternative theories of vision in a manner that seems incompatible or even contradictory to a modern mind.
2. Extromission Theory
On W19147-8v (K/P 22v, 1489-1490) Leonardo refers to combined extromission - intromission theory:
He pursues this theory on CA138rb (c. 1490):
On CA270vc (c. 1490) he explores this combined extromission - intromission theory at length in a scholastic fashion with a series of arguments and counter arguments, beginning with a general statement:
He now gives an:
Example.
One of the chief criticisms levelled against emanation theories of vision had been that the emanating object would gradually exhaust its resources. Leonardo's examples are clearly intended to counter this objection or as he himself puts it, these examples are:
To confute:
A further illustration follows to challenge those who exclusively hold an intromission theory:
Example:
He goes on to cite other cases which are almost certainly copied from an as yet unidentified mediaeval source:
Examples.
Moreover, it is said that the wolf has power in its gaze to make men have hoarse voices.
The ostrich /and/ spider is said to hatch its egg with its sight.
Maidens are said to have the power in their eyes to attract the love of men.
He continues with this theme of extromission on CA270vb:
Having presented cases in support of extrmoission, Leonardo considers cases in support of intromission beginning with two examples involving a camera obscura:
How bodies...send...their form and colour and power outside themselves.
Second example.
As noted earlier (see above p. ) he illustrates such a box camera obscura on A20v (fig. 679, 1492). On CA270vb he also provides a:
Third Example.
This principle he illustrates on CA125vb (see above p. and fig. 694). In the final passage on CA270vb and the opening paragraph of CA270rb he returns to the theme of how objects can emanate images without exhausting themselves:
Quality of the sun.
He now gives his reason for citing these examples:
Opinions.
A scholastic type of discussion ensues with arguments in support of intromission:
Contrary opinions.
An objection from the extromission camp follows:
Confutation.
But in the next paragraph this objection is, in turn, dismissed such that the discussion ends in favour of the intromission theory:
Proof /to the/ contrary.
Hence Leonardo's only extended discussion of an extromission theory, ends with a defence of an intromission theory. His later notes leave no doubt that he favours a strict intromission theory of vision.
3. Intromission Theory
From the time of Pliny5 supporters of the extromission theory had cited the example of nocturnal animals such as cats to support their claims. On CA90rb (c. 1490) Leonardo challenges such examples in order to establish a strict intromission theory:
On the nature of vision.
A standard argument cited against extromission theories had been a time factor. If something is emitted from the eyes it would take time to reach the object and return to the eye.6 Yet we see objects without a time lag. Leonardo uses this objection on A81r (1492) to argue for a strict intromission theory:
In his arguments for intromission Alhazen had cited the example of after images.7 Leonardo uses this example on CA204ra (c. 1490):
On CA204rb (c. 1490) he cites another demonstration to prove that the eye sees through intromission:
On A78 (1492) he notes: “Since effects often show the nature of their causes, I shall describe the nature of the eye with these and in what manner it receives within itself the species of objects.” In the passage that follows he uses the experiment of a pushed eyeball to illustrate refractive properties of vision (see below p. ). Filarete, in the section of his architectural treatise devoted to perspective, had made an analogy between eye and magnets.8 Leonardo uses the same analogy on CA109va (c. 1490): "The air, the moment it is illuminated fills itself with infinite species of which the eye makes itself a magnet." He develops this analogy on A27r (1492) under the heading of:
Perspective.
In addition there are various passages where an intromission theory is implicitly assumed. On A36v (1492), for instance, he points out "All things send their similitudes to the eye through pyramids. On CU23 (TPL15, c. 1492), while comparing poetry and painting he again alludes to the intromission principle:
Another passage devoted to the comparison of poetry and painting, on CU21 (TPL2, 1500-1505) again alludes to intromission:
Further allusions to intromission occur in his anatomical writings. On W19019r (K/P 39r, c. 1489-1510), for instance, he points out:
On W19047v (K/P 48v, c. 1489-1510) he refers again to intromission:
The theme is broached anew on W19045v (K/P 50v, c. 1489-1510):
On W19038v (K/P 80v, c. 1489-1510) intromission is again alluded to briefly: "the object moves the sense." The intromission question is broached at greater length on CA345vb (fig. , 1505-1508):
Whether or not a spirit can see bodies, not having to /do other/ than receive their species.
On CA345rb (c. 1508) he restates this idea as an assertion: "One does not see anything which does not send its species through the air." Seen as a whole these passages leave no doubt concerning Leonardo's position. In the early period he is aware of and cites various arguments in favour of the extromission theory of vision. But these he rejects and champions instead the intromission theory.
It is easy to claim that images come to the eye. But what happens when they reach it and how do they pass from its surface to the brain?9 Leonardo has at least three basic solutions to this problem. He begins with an idea that the images converge to a single point and are there comprehended somehow by the visual power. At a second stage he considers the possibility that images converge to a point, intersect and then diverge again. This idea he at first dismisses because it would imply that images end up inverted within the eye. At at third stage he accepts that images are inverted at the pupil and asserts that they are inverted a second time by the crystalline lens. In his late notes he devises two clear demonstrations against the single point theory. We shall consider each of these stages in turn.
4. Point Theory (Stage One)
In five passages Leonardo adopts the traditional theory that images converge to a point in the eye. Four of these passages are in Manuscript A (1492). A first reference occurs on A3r. Having defined "perspective," "pyramidal lines" and "point," he notes that "this point is that which, situated in the eye, receives all the points of pyramids in itself." On A10r he reformulates his basic definition of perspective and pursues the notion of a point in the eye:
An objection is now mentioned:
This proof begins with a diagram illustrating his opponent's argument (fig. ) which Leonardo then explains:
He now draws his pyramidal solution to the problem (fig. ) which he explains in the text below:
Let ab be the eye. Its centre touches the point mentioned above. If the line ef is to enter as an image through such a small aperture of the eye, it must be confessed that a small thing cannot enter into a smaller th ing unless it diminishes and if it diminishes it is fitting that one attributes /this to/ a pyramid.
The visual pyramid thus accounts for how large objects enter a small eye. On A36v this theme continues. He notes that the vanishing point will never be higher than the eye:
On A37v he pursues the problem:
But here he is citing an idea from Euclid's Optics (see above pp. ). He himself had, in the meantime abandoned the point theory.
5. Possibility of Intersection (Stage Two)
By 1492 the term point has taken on another meaning for Leonardo. It is now potentially the point where lines of a converging pyramid intersect before diverging anew in intverted form. But if the iamges are inverted, why should the eye see them right side up? This difficulty troubles him and his first response on A77 91492) is to reject the point theory in a passage entitle:
In the visual power the species are now reduced to percuss at a point.
Having rejected the theory on logical grounds, he provides a demonstration to prove that images cannot terminate at a single point:
Directly beneath he draws a diagram (fig. ) and then explains why a piece of straw is experienced in this way:
He now interjects an opposing opinion and refutes it:
Here it is logic that persuades him that there should be no intersection of images within the eye and this logic prevents his approaching the problem of image formation int erms of physics.
6. Intersection and Divergence Beyond the Point (Stage Three)
Even so, by 1490, he has begun to consider seriously an alternative that images intersect, diverge, only to converge and intersect again. Two analogies persuade him to accept this possibility: one between the pupil and the camera obscura; a second between the crystalline lens and a sphere of water. This he illustrates on CA222va in three diagrams (figs. ), accompanying which he notes:
Here he accepts that images converge at a point, but now intends it in the sense of point of intersection. Similarly on Forster III 29v (1493) he states that "similitudes of bodies intersect in a point." The accompanying diagram (fig. 689) makes it clear, however, that the images subsequently diverge.
This more complex meaning of point is also found in later passages. Hence one reason for his extended eulogy of the point on CA345vb (see above pp. ) is because images "can be reborn in such a small space and recompose in their dilation." And on W19149-52r *K/P 118r, 1508-1510) although he asks: "How do we conclude that the surface is reduced to a point?", the accompanying diagram (fig. 1120) reveals that the images diverge immediately beyond. Thus, after 1492, when he refers to images coming to a point, he frequently means something very different than in his early notes.
This new concept of double inversion of images within the eye is illustrated in a number of diagrams. ON BM171v (1122, c. 1492) he is exploring the idea. On CA337ra (figs. 1144-1145, c. 1492*) his analogies between both pupil and camera obscura and crystalline lens and sphere are explicitly shown. These analogies underlie his later drawings on CA345vb (fig. 1150, c. 1508), K/P 118 (figs. 1154-1156, 1508-1510) and a series of alternatives considered on D2v, 3r, 3v, 8r, 8v, and 10v (figs. 1172-1178)
*Pedretti, 1979, claims c. 1490. Since Leonardo explicitly denies the possibility of inversion on A77, (1492) it is unlikely that he would have made careful demonstrations of such inversion prior to this.
Two experiences prompt this view. One is the camera obscura (see above pp. ). A second is refraction which leads him on BM221v (1500-1505) to claim that: “The concourse of lines created by the species of objects positioned in front of the eye do not converge to a point within this eye along straight lines.” In addition he develops two demonstrations to counter the point theory.
7. Small Objects in Front of the Eye:A First Demonstration Against the Point Theory (Stage Four a)
As early as 1490, on CA250va, Leonardo is exploring the how objects smaller than the eye are perceived, (see below pp. ):
In defence of this claim a demonstration (fig. ) follows:
This leads him to conclude:
He reformulates this demonstration in terms of an attack (fig. ) on the point theory:
He returns to this theme on K125/45/r (after 1504). Here he is content merely to draw the diagram (fig. ) and note that objects smaller than the pupil do not occlude objects beyond them. On CA237ra (c. 1500) he had redrawn the basic diagram (fig. ) and implicitly attacked the point theory:
The species of an object less than the eye do not converge pyramidally in this eye.
He attacks the point theory briefly on F28v (1507) (see below pp. and then at greater length on D6v (1508) where he begins with a general statement:
Of the human eye.
An adversary's claim is now introduced and countered:
The attack continues with an appeal to experience:
Another example follows:
He illustrates these situations in the right-hand margin. In the upper diagram (fig. ) he shows how a small object in front of the eye does not occlude objects behind it. The caption: "author," identifies this as his position. In the next diagram he shows how a small object occludes things beyond it if one assumes a point theory of convergence. The caption: "adversary" identifies this as the competing point of view. On E15v (c. 1513-1514) he returns once more to this demonstration:
On the eyes.
Here the text stops and we are told to "read in the margin," where the passage continues:
8. Apertures and Contrary Movement of Objects: A Second Demonstration Against the Point Theory (Stage Four b)
Meanwhile, he had been developing a second demonstration against the point theory involving apertures and the contrary movement of objects. At first he merely sketches the problem in passing on CA112ra (1505-1508). Then he becomes puzzled by the phenomenon as is clear from a note on CA222vc (fig. , c. 1506-1508):
He pursues this problem in the Manuscript K. On K127/47/v he begins by considering a stationary situation (fig. ):
On K127/47/r he introduces the factor of motion into the discussion: “It is also possible that a same pupil sees a same object at a same (object) time make 2 contrary movements without alteration of this pupil.” Beneath this he draws a diagram (fig. ) which he explains:
This explanation continues on K126/46/(v):
On K125/45/v he provides further details concerning the experiment:
In cases of motion...of the object between the eye and the aperture of the paper you need to make the perforations with very minute apertures and draw the thing that moves which is slender as the point of a needle and in moving let it touch your eye brows and let the paper be moved to a distance 1/4 braccio from the eye and through the apertures one sees the air.
On K126/46/r, he adds: “But if the motion of the object be above the perforated paper then the eye will see the true motion of the object.” Beneath this he adds a diagram (fig. ) and a text "let us say that a moves through abc: which then breaks off. On D2v he returns to this problem:
Of the human eye.
This process he illustrates in the upper right-hand margin (fig. 1142) and then describes in the text:
Beneath the first diagram in the right-hand margin Leonardo redraws a detail (fig. 1143) followed by a caption "crystalline sphere placed in the centre of the eye" and a marginal note: “Here it is presupposed that the pupil has visual power in every part of it side and this is so in effect and without this every demonstration is ruined.” Below this he draws an other diagram (fig. ) which he numbers "2e" and then describes in the accompanying text headed:
On D4v, this experiment is used to attack the point theory. Under the heading: "Eye of man" he begins with a claim:
A draft description of his needle experiment follows:
Why this should happen the explains:
He now reformulates this experiment (fig. ) in greater detail:
In the right-hand margin he draws three diagrams: one (fig. ) which merely shows the inversion principle of all apertures; a second (fig. ) which illustrates the experiment just described and a third (fig. ) which shows the adversary's claim outlined in the marginal note beneath:
This demonstration against the point theory helps him to account for the unclear boundaries of nearby objects (see below pp. ).
9. Single or Double Inversion in the Eye
If vision does not occur through images coming to a point, how is it that the eye sees? Among Leonardo's earliest explanations of this problem is a passage on CA85va (fig. , 1503-1504), where he asks:
In what way the eye sees the things placed in front of it?
Accurate vision, he claims, is only possible along the central line of sight:
He supports this claim with a demonstration familiar to the optical tradition:
Here Leonardo appears to accept a theory that images concerge to a point but at the same time emphasizes the role of a central ray (see below pp. ). If the central ray along is held responsible for vision, then it is not necessary to consider the inversion of images and there is no problem of explaining why the eye should see inverted images as being right side up.
By 1492, however, he is exploring an alternative, namely, that images are inverted twice within the eye. Two analogies underlie this new theory. He notes that both a camera obscura and a sphere of water invert images. By anlogy, he assumes that images are inverted by the pupil, reinverted by the crystalline lens and are therefore seen in an upright position. The explanation furnished by these two analogies is basically simple. Even so Leonardo is not content with a de fgacto answer. He is continually changing his mind about precisely where in the eye the two inversions take place. For this reason it is necessary to examine in detail his various notes concerning double inversion within the eye.
On BM171v (1492), for instance, he draws a sketch how light is inverted in passing through an aperture (fig. 1121) and directly beneath he illustrates an analogous inversion (fig. 1122) when images pass through the aperture of the eye. In the text opposite he discusses the camera obscura principle:
What happens to images once they have been inverted by a pupil, which functions as a camera obscura, remains a problem for him. In his diagram (fig. 1122) he shows how the rays diverge and then, for no apparent reason, begin converging again until they interesect a second time at the posterior border of the eyeball, before entering the imprensiva. In a third diagram (fig. 1123) he draws a tube-like nerve which extends to the pupil. This idea he develops in a fourth diagram (fig. 1124) in which the nerve is expanded to show a series of inversions caused by continual refelction. The nerve continues beyond the eyeball and connects with a container marked imprensiva. Although the location of the imprensiva is clearly marked in the diagram, Leonardo's text confirms that he is troubled concerning its precise whereabouts:
What is seen.
In the course of 1492 he studies more carefully physical models which could account for the double inversion that he beleives occurs in the eye. Hence on CA125vb he explores both the inverting properties of a camera obscura (fig. 694) and spheres of water (figs. 1126-1132). On CA125va this theme is pursued. In the upper right-hand corner he again draws a sphere of water (fig. 1139) beneath which he explains:
In the lower right-hand column of CA125va (1492) he contracts the perception of far off and nearby objects:
Alongside this passage he draws a diagram (fig. 1138) showing a first inversion behind the pupil and a second inversion behind the sphere of the crystalline lens. On CA222va (c. 1490[P] but probably 1492) this theme is pursued. He mentions how all lines come to a point in the eye and then diverge again (see above p. ), adding:
Beneath this passage, in the right-hand margin he draws a rough sketch (fig. 1151) showing double inversion of images produced ;by a pupil acting as a camera obscura and a crystalline lens as a sphere of water. In the lower right-hand corner he redraws the diagram (fig. 1152) and then once more (fig. 1153), accompanied by a brief text:
And that which falls between angles which are less equal, is less seen, as ac.
On CA337ra (fig. 1144, c. 1490) he develops his analogies between both pupil and camera obscura and between crystalline lens and a sphere of water. Here the drawing is so precise that it has the appearance of an actual model. Had he actually placed his eye in front of a ball of water in a camera obscura, however, he would have experienced only a single and not a double inversion of images.10 In the upper part of CA337ra (c. 1490) he integrates his double inversion principle within a schematic adaptation of the eye (fig. 1145).
On CA345vb (c. 1508), where he also discusses the camera obscura (see above pp. ), he returns to this problem of double inversion in the eye. Under the heading: "Whether or not the spirit can see bodies, having only to receive their species" he again draws the crystalline lens in the form of a sphere of water (fig. 1150) beneath which he explains:
To the left of this passage he sketches three further visual hypotheses concerning the path of rays within the eye (figs. 1147-1149). In a first sketch (fig. 1148) some of the rays are inverted just beyond the pupil and then stop short at the crystalline lens. Other rays pass directly through the eye without inversion. In a second sketch (fig. 1147) directly beneath this he again shows some of the rays as being inverted while others penetrate the eye without inversion. A third sketch is more developed (fig. 1149). Here he shows how some rays come from an object, are refracted by the cornea, pass through the pupil, are inverted, refracted a second time at the crystalline lens and then converge to a point. In addition he draws other lines directly from the object to this point of convergence in the eye, probably to contrast the geometry of the situation with the realities of refraction. A note directly beneath this sketch confirms that Leonardo intends to study this problem mroe carefully in terms of anatomy:
He pursues the question of double inversion in the eye on W19150v (K/P 118v, 1508-1510):
Hence it is possible that the entire circumference of a circle sends its cimilitudes to its intersection as is shown by the fourth of this which states that all the minimal parts of species penetrate one another without occupation of one another.
In the right-hand margin he writes "These demonstrations are as an example of the eye," and sketches a first inversion caused by the pupil (fig. 1154). Below this he makes a more detailed drawing (fig. 1155) showing how an object ab is first inverted by the pupil at c and then by the crystalline sphere at q in order to appear right side up at rs. A summary text follows:
Below this he draws another rough sketch (fig. 1156) of double inversion within the eye, and three sketches which appear to show the refractive properties of a flat (fig. 1014), convex (fig. 1015) and concave surface (fig. 1013) respectively. He also illustrates the inversion principle within a camera obscura (fig. 705, see above pp. ).
10. Models
As early as 1490 Leonardo had become concerned with producing physical models of the eye. On CA222ra, for instance, (figs. 1157-1161) he notes:
A diagram on CA141rbc (fig. 1164, c. 1490) might well show a model eye. By 1508 he returns to the problem. On CA190vb, for instance, he drafts two diagrams of the eye (figs. 1074-1075) accompanying which he notes:
"Make such an eye large and of glass like a natural one."
In the Manuscript D he goes further. In a marginal note on D2v (1508) he recommends:
Preliminary instructions concerning such an instrument are given on D7v in the lower right-hand margin (fig. 1165):
In the upper portion of this margin heoutlines further possibilities:
This he illustrates in a diagram (fig. 1168) showing a man with his face immersed in such a mask with water. He also notes: "And if you wish to see with a single eye make it with the body of a small or large ampulla, etc." To illustrate this alternative he draws two further diagrams. In the first of these (fig. 1166) an eye is immersed in "tepid water" and as the caption explains: "Here the air makes itself a concave mirror." In the second diagram (fig. 1167) the form of the ampalla is reversed. As the caption explains: "here the air makes itself a convex mirror." The idea of a mask of water, mentioned on D7v is developed on D3v (fig. 1169) under the heading:
To make an experiment how the visual power will function as instrument of the eye.
He illustrates this model in the right-hand margin (fig. 1169) and beneath it adds the caption: "Tinge the larger glass /sphere/ on the outside and you will make the uvea." In his model he requires that the pupil be "an aperture...four times as large as is the pupil" but then adds "or approximately which makes no difference." The model is clearly intended to simulate only the inversion he assumes occurs in the crystalline lens. Hence he sees no contradiction between the single inversion shown in this diagram and the double inversion in the drawing of the eye alongside (figs. 1169, 1171).
While fascinated by models, he does not insist that they should give a perfect simulation of reality. This separates his attempts from the model eyes of seventeenth century individuals such as Scheiner and for the experimental method codified by Galileo, Huygens and Newton. Even so Leonardo's efforts are of great significance because they shift questions of the visual process, from the realms of abstract philosophical debate to the domain of concrete physical problems which can be reconstructed through models and simulated experimentally. For this reason it is useful to trace the development of his ideas in detail. Directly beneath his model eye on D3v he draws a conceptual version (fig. 1171) which he carefully letters and then describes in the text alongside:
By now he is convinced that double inversion is the only means of accounting for the visual process. Beneath his diagram (fig. 1171) he adds a marginal note pertaining to anatomy:
He is still not content with his model of the eye and in the lower right-hand corner draws further diagrams (figs. 1170, 1172). In a first draft (fig. 1170) he increases the size of the crystalline lens relative to the sphere of the eyeball. In his second draft (fig. 1172) he carefully marks each of the parts with letters but then makes no reference to these in the text that follows:
In the examples considered thus for Leonardo has assumed that a first intersection occurs at the pupil and a second intersection at the centre of the crystalline sphere - which he sometimes terms vitreous body. In a larger diagram near the centre of D3v (fig. 1175) he considers a new alternative. As in previous cases a first intersection at c is caused by a pupil which functions as a camera obscura. The rays ov and or then diverge until they arrive at the crystalline lens where they are refracted to a converging path. On emerging from the lens at p and q, the degree of convergence is again increased due to refraction and the rays cross a second time at n just prior to impinging on the sides of the optic nerve at h and k. To the left of this diagram he adds a short note:
(* reading li e dato instead of lietato.
Beneath this diagram on D3v a further explanation follows:
How the species give themselves to the visual power by two intersections of necessity.
He now draws a bracket to indicate the end of a paragraph and considers the arguments of an adversary:
On D3r he pursues this theme of two necessary intersections within the eye. In the upper right-hand margin he draws a more detailed diagram (fig. 1174) which he explains in the text alongside:
There follows an explanation how refraction delays the intersection of the rays:
Next he describes the second intersection in the eye:
In the right-hand margin he draws another diagram (fig. 1174) with an alternative set of intersections. As he explains this is:
A second opinion with the same two intersections.
At this point he draws two strokes (//) to indicate a new paragraph:
The explanation on D3r involve alternatives which Leonardo implicitly rejects. He had put forth his own explanation of the visual process on D3v. To this he returns in a diagram (fig. 1176) on D8v. Here there is no explanatory text. But on D8r he redraws the diagram (fig. 1177) and adds the caption "sc is 1/3 of md," which refers to the size of the image emerging from the crystalline lens. On D3v he had referred to the same ratio. Beneath the diagram and caption on D8r, he adds a marginal note in which he dismisses the alternative explanations which he had considered on D3r:
In the main text he gives his own version of the visual process:
He interjects an explanation why refraction occurs at the lens:
His explanation of the visual process now continues:
To clarify the nature of the first intersection in the eye he draws a camera obscura in the lower right-hand margin (fig. 698) which he explains in the main text:
How the species of objects received byt he eye intersect within the albugineous humour.
On D10v he takes the next logical step, now integrating his drawing of the camera obscura on D8r with his model of the eye with two of its inversions (fig. 1178). On D10r Leonardo had included another modification. Instead of depicting the crystalline lens as a perfect sphere, he had removed a lunule from the posterior section (fig. 1076). This modification is included in his diagram on D10v (fig. 1178). This series of diagrams in the Manuscript D marks his last recorded attempts to explain the inversion of images on the eye. After 1508 he does not return to questions of the visual process.
11. Conclusions
Leonardo's explanation of the visual process had changed dramatically in the period 1490-1508. At the outset he had favoured two possibilities:
1. that only the central ray was oeprative in vision, and
2. that rays coming to the eye converge to a point.
Each of these alternatives had avoided the embarrassing problem of inverted images in the eye. By 1492, however, his study of camera obscuras and spheres of water led him to explore the possibility that images are inverted twice within the eye.
In his early versions he posits that a first inversion occurs just behind a pinhole-like pupil and a second inversion in the centre of the crystalline lens which, he assumes, is like a sphere of water. In 1508 he considers yet another possibility: that a first inversion occurs in the crystalline lens and a second inversion occurs in the vitreous body beyond the lens. This explanation he rejects also, and in the end adopts a further alternative wherein a first inversion occurs behind the pupil and a second inversion occurs in the vitreous sphere behind the lens.
In the meantime the whole nature of his explanation has changed. In 1490 his explanation is largely in philosophical terms and so formulated that it precludes the role of physical images. By 1508, his explanation is in terms of physical models. The visual process is now a domain of physics. Granted his models may remain imperfect. But the challenge is now there to match the models with visual experience and text claims by experiment. The way is prepared for the model eyes of Scheiner in the next century.