Part Three, Chapter 1: Anatomy and Physiology of the Eye

1. Introduction 2.Vision as the Chief Sense 3. Eyelids and Eyelashes

4. Cornea 5. Aquaeous Humour 6. Iris 7. Pupil 8. Uvea

9. Crystalline Humour 10.Vitreous Humour 11. Retina 12.Orbit

13. Optic Foramen 14. Second or Optic Nerve 15. Optic Chiasma

16. Optic Tract 17. Third or Motor Ocnli Nerve 18. Ophthalmic Nerve

19. Ventricles 20. Visual Power 21. Conclusions

1. Introduction

Leonardo considers vision as the most important of the senses and therefore devotes considerable attention to the eye and its properties. From the outset he is explicit in doubting the verbal claims of the Ancients, preferring instead the visual evidence of experiment. On CA119va (1492), for instance, he notes:

Beginning of Perspective

That is of the function of the eye

This fresh approach to authority which rejects verbal claims and insists on visual demonstrations helps explain why his notebooks have no lists of tunics of the eye as found in Galen, Witelo, Bacon, Pecham or Mondinus. It also explains why many of the questions he raises concerning vision in a long list on CA360rc (c. 1490) are primarily physical and not philosophical (see Chart 19). Even so Leonardo's approach is not as straightforward as one might at first assume. In his anatomical notes he records his intention to study the eye carefully. Hence on W19037v (K/P 81v, 1489-1510), while outlining the contents of his book he notes: "And then perspective through the function of the eye and of hearing, you would say, of music and describe the other senses." In a note on /ca345vb he writes a reminder: “Write in your anatomy what proportion all the diameters of all the spheres of the eyes have and what distance the crystalline lens has from them.” Elsewhere on W12641r (K119/39/r, 1508-1510) he gives even more exact instructions:

Notwithstanding these plans there is no conclusive evidence that he actually made a careful anatomy of the interior of the eyeball. Indeed, his most detailed diagrams of the interior bear striking resemblance to the diagrams of Alhazen, Bacon or Pecham (e. g. fig. 963, cf. figs. 957-962). At the same time, his anatomy of other parts of the visual process such as the orbit, optic chiasma, nerves and ventricles is very painstaking.

One reason for this discrepancy between such careful studies in some parts and negligence of others is that the visual process in the eyeball only gradually emerged as a problem for him. In the early period he considered both intromission and extromission theories. He also believed that the visual power was situated at a point in the pupil. Questions of image formation were thus not relevant. Gradually he discovered that there was not one point in the pupil to which images converged, but that all points could serve this purpose.

Questions on CA360rc Answers elsewhere in Notebooks

How and why many things that

are mirrored come to the eye

upside down.

Why a given thing that is

mirrored appears larger than

it is?

Why when a thing is mirrored

it appears smaller?

Which mirror is that which

shows things precisely /the

same size/?

Which mirror shows things out-

side itself?

How is the mirror the master

of painters?

Why does the eye go changing

from hour to hour, growing

larger and smaller?

Why does the pupil become

smaller the more light it has

in front of it and, conversely,

grow larger in the dark?

Why are the things seen by the

eye...small and appear large?

Why does a thing seen through

a slit by two eyes become

double and contrary: that is,

the thing seen on the right

side goes to the left eye and

likewise that from the left

goes to the right?

Why does a building int he fog

appear larger?

Why does the eye not see perfectly

except in a straight line?

Why the pyramidal lines which

part from the eye make a

point at the thing seen?

Why the said pyramid, depart-

ing from the eyes and making

a point at a thing positioned

in the water, the lines are

bent on meeting the water and

do not maintain their straight-

ness?

Why the things seen only make

a pyramid in the eye?

How the two eyes make a pyramid

at the thing seen?

Which things the eyes can see

even if only half?

Which things the eyes can never

see half?

Many lines near one another

cannot be numbered and do not

touch line for line.

Cutting a pyramid all things

appear to be small although

they are large.

...things seen are all bent

although they appear.

...cut the pyramid to find

the things.

...eye bent.

...to put in a painting the

things seen.

...which parts from the thing

seen and leads...observes

straight lines when they pass

through two...in the air that

is thick and thin...to know

the size of the sun or some

other planet, because it

cannot be measured between

us.../without the/ action of

the air, that is thick and

thin.

Through his studies of the camera obscura he stablished, moreover, that this point was physical and that image formation was also physical. If the pupil acted in the manner of a camera obscura and inverted images within the eye, how was it that images are, nonetheless, seen right side up? Image formation was not a problem of physics. Leonardo began looking for physical conditions to account for a second inversion. He noted that spheres of water inverted the image. The crystalline lens, he reasoned, was like a ball of water. And just as he studied camera obscuras to simulate the pupil, he examined spheres of water to simulate the crystalline lens.

There remained a flaw in Leonardo's approach. His models of the eye were based not on the structure of the eye itself, but on what logic led him to assume must be the structure of the eye, and this limited the value of his results. More important than these results, however, was the shift in approach that he introduced. The visual process which had traditionally been a purely philosophical problem was now a question of physics, involving model-making.

Our examination of Leonardo's notes on vision will open with his comments in praise of the eye. His description and anatomy of individual parts will lead to physiology of parts of the eye, for example, how the eyelashes reflect, how the cornea mirrors and refracts, how the uvea reflects and so on. His theories concerning extromission and extromission will then be examined. An analysis of the experiments that led him to abandon his point explanation of vision will follow, and lead to consideration of his theories concerning double inversion of images within the eye. In a subsequent chapter his notes on visual appearances and illusions will be listed and compared with those in Euclid's Optics. A study of his notes on optimal and minimal conditions of vision will complete this section.

MODERN TERM LEONARDO'S TERM TIMES TIMES

eyelid palpebre 15

eyelash coperchio 24

cornea (pupil) luce 70

cornea (pupil (?) cornua 2

pupil (cornea) popil(1)a 253

albuminoid humour/ omore/spera 15

sphere albuginea

crystalline humour/ omore/spera 23

sphere cristallina

vitreous humour/sphere omore/spera vitrea 16

uvea, grape-like tunic uvea 15

visual power virtu visiva 70

potentia visiva 5

imprinting power virtu imprensiva 2

orbit --

optic foramen buso della chassa dell'

occhio

optic chiasma --

optic nerves nervi ottici 21

ventricles ventrichuli

senso commune senso com(m)une 25

imprensiva imprensiva 43

judgment giudizio 5

parte giudiziale 1

virtu giudiziale 1

memory memoria

2. Vision as the Chief Sense

Leonardo considers the eye alternatively as a better sense (CU28, TPL27, 31), a nobler sense (CU31, 29, 40, 28 TPL14, 20, 21, 27, 31, C. 1492; 28, 34, c. 1500 Mad II 62r, 66v, 1503-1504) and as the noblest of the senses (A99v, BN 2038 19v, 1492). The primacy of sight had been emphasized by both Aristotle¹ and Cicero². Leonardo provides a series of fresh arguments to defend this claim. Some of his arguments are aesthetic. Vision, he claims is the more immediate (CU40, 42; TPL21, 23, 1492) and the more eternal sense (CU40, 42, 12, 39; TPL21, 23, 24, 29, 1492; CU13, TPL16, 1500-1505: W19101r, K/P 197v, c. 1510-1513 or 1515) and associated with divine proportion (CU28, TPL27, 1492), divine proportionality (CU41, TPL32, 1492) and divine harmony (TPL32). Developing ideas also found in Aristotle³ and Cicero⁴, Leonardo claims that the eye is a window of the soul.⁵

In addition he mentions scientific arguments why vision is the nobler sense and painting the nobler art. The poet, he argues, copies words which are the works of man. The painter copies the visible works of Nature which are the works of God (CU31, TPL14, 1492). Painting goes to the senso commune via the same sense of sight as does the original object: poetry does not (CU23, TPL15; CU40, TPl21; CU42, TPL23, c. 1492; CU17, TPL7, 1500-1505). Painting is more useful and more communicable (CU17, TPL7, 1500-1505). Painting has more conformity with the thing imitated (CU433, TPL411, c. 1492). It imitates with more truth (CU31, TPL14, c. 1492), with more truth and certainty (CU17, TPL7, 1500-1505): with such truth as is possible in Nature (CU41, TPL32, c. 1492) indeed, with the first degree of certainty given its link with mathematics (Mad II 67r, 1503-1505).6 At the conclusion of his imaginary discussion between a poet and a painter on CU34, (TPL28, c. 1500) he launches into a long eulogy of the sense of sight:

A description of the range of sight follows:

Date Passage Term

Coperchio Coperchio di sopra

Coperchio di sotto

1492 1 3 3

1503-1504 Mad II 25v 1

1508 F37r 1

F29v 1

D1v 5 2 2

D2r 1

D9v 2 1 1 (1)

___ ___ ___

10 8 6 24

Palpebra

1508 F30r 2

F29v 4

F29r 3

D1v 5

D4v 1

_____

15

3. Eyelids and Eyelashes

On at least 24 occasions Leonardo uses the term coperchi to refer to both eyelids and eyelashes. He also uses the term palpebre in this double sense at least 15 times. (Chart 21). On rare occasions, such as F30r, he refers specifically to the eyelashes as labri. His rough drawings of eyelashes begin towards the mid-1480's (figs. 965-968, cf. 969-972). Most of his more careful drawings of this part of the eye date from 192 and 1508 when he explores their function.

3.1 Function

For Leonardo eyelashes function as mirrors that reflect images and produce the rays which appear to surround heavenly bodies. Eyelashes thus play a role in his explanation of the optical process and help to account for certain astronomical illusions (see below pp. ). His earliest extant drawings of this reflection at the eyelashes occur on CA141vbc (figs. 982-985, 1490). These are without text. On CA125rb (c. 1490-1492) he again draws reflecting eyelashes (fig. 978) which, as the caption explains, show: “ the reason why the rays of the cornea (luce) appear to the eye and why upside down and how the rays are generated in the thickness of the lashes of the eye.” On CA125va (fig. 980, c. 1490-1492) he describes an experiment in this connection:

He explains how this contrary movement can be connected through a ball of glass corresponding to the sphere within the eye, and the reformulates his:

Experiment how, not moving the cornea (luce) from its site, the objects seen by this appear to move outside their place.

The opinion of an adversary is now introduced and countered:

Two conclusions follows: “Hence the pupil (luce) does not move in its position. How the above-mentioned things show that the pupil inverts that which it sees.” Directly beneath the drafts a rough diagram (fig. 977), which he crosses out and redraws (fig. 987) adding a caption: “ If you place a finger at abin such a way that the ray made in the eyelash below does not see the pyramid of the rays made by mn, it will be lacking to the eye.” He pursues his study of the role of eyelashes on BM115v (1492-1497) in a passage entitled:

Of the light which appears in the eye in its friction.

The role played by eyelashes is considered at length in both the Manuscript F and the Manuscript D. On F37r (1508) he begins with a general proposition: “Many are the times that the simulacra...of a given luminous /body/ will be two or three times at the same time in a same eye.” In the right-hand margin he draws two diagrams (figs. 986-988). These he marks a and b respectively and then explains in the text alongside:

On F30r this discussion continues (fig. 990):

An explanation of the diagram now follows:

Here the folio ends. The passage continues on F29v opposite:

He now introduces the objection of an adversary:

On F29r he provides further evidence against the adversary's proposal:

A further demonstration follows:

He adds detailed instructions concerning procedure:

Below this he draws another diagram (fig. 991) showing rays coming from above and being reflected from the lower eyelash to the upper lid and into the eye. A note on the apparent size of distant and nearby objects follows:

In the right-hand margin he pursues this theme with a diagram (fig. 999) and a further note:

He also considers these problems on three folios of Manuscript D. On D1v, he begins with a general heading: "Of the eye" and the subheading: "Why the rays of luminous bodies make themselves larger to the extent that they are more remote from their source." A general proposition follows: "The rays of luminous bodies grow the more they are removed from their sources." This he illustrates with a diagram in the upper right-hand margin (fig. 993, cf. 992) which he then explains in the text proper:

Consideration of attendant circumstances follows:

The folio ends with afterthoughts how the demonstration might be presented more clearly:

Now a demonstration such as this should be divided into parts to make it better known, placing first its conceptions and other prpositions necessary for a similar proof etc.

In the right-hand margin of D2r he outlines an experiment to show that rays surrounding bodies are due to an illusion within the eye:

If you wish to certify how the rays of luminous bodies are caused by the images of such bodies which impress themselves in the thickness of the eyelids where they are reflected to the pupil of such eyes, then open the eye to the extent that the pupil does not see these eyelids and you will see the lights without rays.

This idea he pursues on D9v beginning with a paragraph under the general heading:

Of the eye.

Leonardo's theory is that the rays which appear to surround stars and other heavenly bodies have their origin in the eye, as becomes clear from the following paragraph headed:

Why luminous bodies show their boundaries filled with straight luminous rays.

Persuasive argument is followed by reasoned explanation:

By way of support he uses an experiment also described in the Manuscript F:

The passage ends with the opinion of an adversary:

The adversary says that a ray bends because it goes to the sense from the rare to the dense.

In the right-hand margin these various reflections of rays are illustrated and further described by captions. Beneath the uppermost diagram (fig. 997) he writes: "This eye sees the ray of the luminous body above." The second diagram shows the opposite situation (fig. 998) as its caption indicates: "This 2nd eye sees the luminous...rays of the luminous body below." The next diagram (fig. 995) considers a further alternative: "This 3rd eye sees...the superior and inferior rays, above and below the luminous rays." Further diagrams, without captions follow (figs. 994, 996). Here he is clearly exploring the limits of the visual field within which reflection from both eyelids is possible.

In Leonardo's interpretation eyelids and eyelashes function as mirrors that produce the rays which appear to surround heavenly bodies and as such account for certain illusions relating to astronomy (see below pp. ).

2. Cornea

Leonardo tends to refer to the cornea as the luce. On CA270rb (c. 1492), for instance, he mentions "the circle of the cornea (luce) which appears in the middle of the white of the eye...." On CA85va (c. 1503-1404) he refers to the luce as "that smaller part of the sphere which is divided by the line st.” The accompanying diagram (fig. ) leaves no doubt that the cornea is intended. Even clearer is his description on K119/39/r (1504-1509): “the pupil of the eye is situated in the middle of the cornea (luce), which cornea (luce) stands in the form of a portion of a sphere which, in the middle of its base receives the pupil....” On some occasions diagrams with captions (figs. ) leave no doubt that luce can mean cornea.

Nonetheless, the term luce, which occurs at least 70 times in the extant notes (Chart 22) remains problematic because it is also used in the sense of "pupil" or "a larger part of the eye." On W19030v (K/P 72v, 1489-1510), for example, he mentions "the luce or the pupil." On CA250va (c. 1490) he refers interchangeably to the space between the pupils (interpopille) and the space between the luce (inter luce) and on A100v (1492) he uses the term "little luce" as if he meant "little pupil." On D3r, by contrast, he refers to the luce as being "the entire sphere labv" (fig. ) which effectively embraces even the crystalline sphere.

On A77 (1492) he uses the term cornua but in such a way that it appears to mean "vitreous humour." He provides no detailed anatomical description of the cornea. Nonetheless, he draws it on various occasions (figs. ) and is interested in the role of the cornea in the refraction and reflection of rays.

4.1 Function

His first extant hints of refraction at the cornea are in two rough sketches on W12692r (figs. 1002-1003, 1487-1493). On W12447v (K/P 33v, 1490-1493) he asks:

Whether images have their proper position in the eye or not?

Whether rays enter the eye /directly/ or whether they are bent at the entrance or not?

On CA141vb (figs. 1016-1019, 1490), a related sheet, he pursues this theme. Accompanying a first diagram (fig. 1016) he adds the caption: "This eye sees a ray departing from the light and it goes below." Alongside a second diagram (fig. 1017) he writes: “This eye sees the light which is mirrored in b in the middle of 2 rays, one which goes from the cornea (luce) upwards and the other which goes from the cornea (luce) downwards.” He then draws a third diagram (fig. 1018) accompanying which he notes:

This sees a ray which parts from the cornea (luce) and goes upwards and

this occurs because that which goes from the rare to the dense...

bends its straightness on entering the dense.

On CA144vb (figs. 1011-1012, 1490) this refraction at the cornea is again suggested and on CA222ra, cf. figs. 1009-1010, 1492) its necessity is asserted:

Witelo, in his mediaeval optical treatise had made an implicit comparison between the arrows shot by archers and visual rays.⁶ On Forst II 69v (c. 1495) Leonardo adopts this comparison to illustrate refraction in the eye: "I also believe that an arrow, /when/ it hits water obliquely, will bend as does a visual ray." Immediately following he describes an experiment relating to this claim:

He returns to this theme in two sketches on BM221v (figs. 1004-1005, 1500-1505) which he develops on BM220r (figs. 1006-1007, 1500-1505) when he compares the effects of linear perspective and visual perception (see Vol. 1, Part 3.3) and notes: “ if the jugment of the eye is inside, the straight lines of the species are bent on its surface, because they go from the rare to the dense.” He pursues this question of discrepancy between the size of objects in visual perception and linear perspective on D2r (1508) under the heading:

How the straightness of the concourse of the species is bent on entering the eye.

An example follows (fig. ):

The objection of an adversary is then raised and dismissed, with the help of his visual ray - arrow analogy:

In the Arabic tradition a device had been developed to measure the amount of refraction in passing from air to water.⁷ Leonardo adopts this as his own in a diagram on F33v (fig. , 1508). He provides further demonstrations of refraction at the cornea in a series of sketches inw hich he explores the limits of the visual field on I35r (fig. 1001, 1497-1499), I46r (fig. ) CA345vb (fig. , c. 1505-1508); F40r, 34r, (figs. , 1508); D1r, 8r (figs. , ); K/P 115r, 118r (figs. , 1508-1510); K118 /38/r (fig. 1020, 1509-1512); CA42va (fig. 1008, C. 1510) and CA385vc (figs. , c. 1513).

On D2r (1508) he again broaches the problem of refraction at the cornea asking:

This explanation he develops on D7r (1508) under the heading:

He is equally interested in the reflection of images from the surface of the cornea. On CA309rb (c. 1490), for instance, he asks:

Why the eye when it sees things on its small surface these appear large to it?

His theory that the eyelids and the surface of the eye function as mirrors, lead him to study the properties of mirrors with respect to vision. Hence on CA125rb (c. 1492), immediately following his discussion of eyelids/eyelashes (see above ) he draws a sketch (fig. 1023) of reversal in a mirror which he then explains:

On CA141vc (1490-1492) he pursues this comparison between images in a mirror and at the surface of the eye:

A diagram (fig. 1019) in the right-hand margin may have been to illustrate this. In a marginal note he considers a related example (fig. 1027):

This theme of images mirrored in the eye of another person is continued on A37v (1492), but now in another context, namely, to help visualize the apex of the visual pyramid which he believes ends in a point within the eye (- an opinion he will later reject):

Well over a decade later he returns to these analogies between the surface of a mirror and the surface of the eye. On CA190vb (1505-1508) he draws a reversal diagram (fig. 1024) beneath which he notes that "the image of the real object changes the right side to the left and the left to the right at the surfaces of mirrors." Lower down, he draws the same reversal principle in connection with a curved surface (fig. 1025), which presumably represents the eye. To the left of this he compares the reversal principle in mirrors with that of camera obscuras.

On F31v (1508) he returns to the question raised on CA141vc, concerning where one sees images mirrored on the surface of the eye (fig. 1022):

In the early period he had drawn an object ab in front of a mirror cd on CA125rb (fig. 1023) to explain the mirroring process of the cornea. On D4r (1508) he redraws this diagram in the margin using the same lettering, (fig. 1026). The elongation of the later figure appears to be accidental. Beneath it he writes a corrective notes: "This figure ought to be square." Alongside the diagram he explains:

Why a mirror changes the sides of an image from right to left and from left to right.

A specific case (fig. 1026) is now considered:

He next introduces an adversary's opinion which he then dismisses:

At the end of the passage this mirroring effect is compared to what occurs when one looks into the eyes of others:

These questions of images mirrored at the cornea help explain his interest in the correspondence between the original arrangement of objects and their arrangement at or in the cornea. On F31v (1508), for instance, he notes:

This he illustrates with a cursory marginal sketch (fig. 1021), in which he draws the eye as a half-moon. Above this he draws two stars as half circles. The star on the left sends its rays to the eye. The rays of the star on the right, impeded by a rectangular object, do not reach the eye. On D2r (1508) he pursues this problem under the headings:

Of the eye

Whether the idola or image has a fixed position on the eye or not.

The images of immobile objects have fixed positions on immobile eyes.

To demonstrate this he draws a diagram (fig. 1028) which he explains in the text alongside:

He now poses the question anew:

In the margin he draws two diagrams (figs. 1029-1030) the lower of which he describes in the text that follows.

He pursues this question of the proportions of images at the eye on D10r under the heading:

On the proportions which the positions of the images have which are impressed on the eye.

He now draws a "1st" illustration which he describes in the text that follows:

On D10r he pursues this question:

Of the proportions which the positions of the images have which are impressed on the eye.

A "1st" illustration (fig. 1034) and description follow:

He goes on to describe a second case (fig. 1033, cf. 1035):

On E15v (1513-1514) he returns once more to this theme:

5. Aqueous Humour

In the early period Leonardo has no specific term for the acqueous humour. On CA270rb (c. 1490), for instance, he refers to it simply as "that water which is in the cornea" (luce). On D2r (1508) he refers to the aqueous humour as the albugineous humour - albeit this term can also mean vitreous humour. In the extant notes there is no record of his studying either the properties or the functions of this humour.

6. Iris

He has no specific term for the iris although he draws it clearly on at least three occasions (see title page to part three).

7. Pupil

Leonardo's term for the pupil tends to be popil(1)a which he uses at least 253 times in the extant notes, (Chart 23). His use of the term is complex, however. On CA309rb (c. 1480), for instance, he describes it as "a concave mirror." On D1r (1508), when he refers to "why Nature made the popilla convex, that is, elevated as part of a ball," popilla appears to mean "cornea" and could even mean "eye." His alternative term for pupil is luce⁸, which is no less complex, luce tends to mean cornea (see above p. ) and can be synonymous with eye (eg. F32r). Hence the terms popilla and luce are both ambiguous and have a range of meanigns including pupil, cornea, and eye.⁹

7.1 Function

In some early notes he describes the pupil as if it were merely an aperture directly linked with the optic nerves (eg. A77, BM171v, fig. ). In other notes (eg. CA125rb), he explores the analogy between pupil and camera obscura, which leads him, on the one hand, to study camera obscuras with apertures of various shapes (see above p. ) and, on the other hand, pupils of various kinds, round, slit-like, etc. (e.g. CA262rd, Mad II2 25r, G44v). In the early period he is interested in pupil size because it provides him with an explanation why distant objects are seen unclearly, as on A100v (BN 2038 20v, 1492) where he analyses:

Why faces in the distance appear dark.

As his studies progress he becomes convinced that there is a direct connection between pupil size and the apparent size of objects. To confirm this he describes no less than five practical and two theoretical demonstrations, some of which recur in an extended passage on pupils on Mad II 25r-27r. He also examines differences between pupils by day and at night and makes comparative studies of pupils in human beings, owls, cats and lions. He refers to the pupils of various owls (common, horned or long-eared, tawny, barn and little), panthers, leopards, wolves, lynxes and Spanish cats. On several occasions he compares the relative size of pupils with the sizes of the imprensiva.

Each of these aspects of his pupil studies will be considered in turn. In a later section it will be shown that he has an ulterior motive for being so interested in this connection between pupil size and apparent size: it has fundamental consequences for his studies of astronomy (see below pp. ). This helps explain why Leonardo writes more about the pupil than any other part of the eye.

7.2 Demonstrations of the Pupil

In order to confirm that changes in pupil size determine apparent size Leonardo devises five practical experiments involving:

1. looking at a star through an aperture;

2. looking at a candle;

3. sudden exposure to daylight;

4. sudden exposure to darkness, and

5. looking through a peashooter.

In addition he describes two theoretical demonstrations. Each of these will be discussed in turn.

Why a thing seems larger when seen by a larger pupil.

In the Manuscript F he uses this experiment to develop his claims concerning pupil size. On F36v (fig. 1036), for instance, he draws a light source at m passing through an aperture op in a sheet of paper ac to the eye at n. Alongside he explains:

F33r; fig. 1040, F32v.

7.2.i Star through an Aperture

On C6v (1490-1491) he describes how a candle 400 braccia from the eye appears much larger than it is until a small stick is placed in front of it (see below p. ). This experience he develops on A64v (1492) into a:

Test to see the true size of luminous bodies.

He returns to this demonstration on CA351vb (c. 1495-1497?) under the heading:

Of the eye and light.

In the above demonstrations connections between size of aperture, pupil and apparent size remain implicit. On D5r (1508) he makes these connections explicit in a passage with the marginal heading: “This occurs because /a/ lesser power has less potential power than /a/ greater...and through this coming...” Here his train of thought breaks off, but on F33r he pursues the problem (see above p. ) beginning with a diagram (fig. 1039) which is a:

In the lower margin he redraws the diagram (fig. 1038) and in the text opposite he continues his explanation:

This amounts to a geometrical demonstration of his claim that the size of the pupil's aperture determines apparent size.

On F32v, the folio opposite, he redraws this diagram (fig. 1040 cf. figs. 1038-1039) and sums up his claim in a marginal note:

Not content with this demonstration involving one star, he drafts a sketch in the lower right margin (fig. 1043) involving three stars. This he redraws at the top centre (fig. 1041) beneath which he asks:

His answer follows:

The accompanying diagram (fig. 1041) appears to show the three stars superimposed on one another. Aware of this ambiguity he drafts a further diagram (fig. 1042) in the upper left-hand corner, with the caption: "the images are not sueprimposed one upon the other." This he restates at the top of F32r: “The images of opaque bodies are not superimposeed upon one another, the eye that judges them remaining without motion.” In the margin beneath this he redraws the diagram drafted on the verso (fig. 1044, cf. figs. 10421-1043) and explains: “In the same mirror or pupil...is the image of all objects positioned in front of it and each of these objects is all in all the surface of the mirror and all in each minimal part of this.” This "all in all" demonstration, (see above p. ) reduces the problem of image formation in the eye to a physical model which can be analysed with geometrical principles. He adds how this can be tested experimentally:

Having compared the pupil with an aperture in a sheet of paper and with a mirror, he now suggests that one can use an aperture in a sheet of paper combined with a mirror to simulate effects in the eye: tracing out the size of stars on the mirror and then, by moving both aperture and mirror, simulate the effects of the eye's motion, each time tracing anew the configuration of the stars seen. He is not content with his diagram, however, and therefore redraws it (fig. 1045) with the caption "this figure is better than the other." In the main body of the text, a full explanation follows:

He is still not satisfied. On F28r he writes: "it is proved here that the visual power is spread throughout the entire pupil of the eye." In the margin he again draws three stars surrounded by a circle pq (fig. 1046), the images of which pass through an aperture ord onto the pupil abcnmo. Below this he develops a diagram he had drawn previously on F33r (fig. 1048, cf. fig. 1039). At the bottom, a further draft (fig. 1047). None of these are explained, however. On TPL628 (1508-1510) he uses his aperture demonstration to explain why the moon appears dull by day and bright by night. His first reason is that colours are better seen when they are in greater contrast to one another (see below pp. ) and then adds:

On TP477b he mentions the experiment again:

Precept

The eye which will have a larger pupil will see objects as having a larger size.

7.2.ii Candle

His second demonstration of the link between pupil size and apparent size begins with a general statement at I19v (1497):

A specific experiment follows (fig. 1049):

On I20r, the folio opposite, he pursues the theme: "The pupil of the eye standing in the air, in every degree of motion made by the sun changes degrees of magnitude," and its converse: "And in every degree of magnitude /of the pupil/ the objects /which are/ seen change to different sizes." Not content, he crosses this out and reformulates it:

A decade later he develops these ideas further on D5v (1508):

At this point he describes his candle experiment:

He summarizes these ideas in two marginal ntoes:

7.2.iii. Sudden Exposure to Daylight

On C16r (1490-1491) this candle experiment recurs in connection with the problem of the eye's sudden exposure to light.

On the eye.

On CA125vb (c. 1490-1492) he again drafts a note how the eye is hurt when suddenly exposed to an intense light, and adds:

This he crosses out and on CA125ra (c. 1490-1492) takes up the theme again:

On H88 /40/r (1494) he cites both the case of sudden exposure to light and the experiment with an aperture:

That pupil will be larger which will see the things of larger shape.

He restates these ideas in the form of brief precepts on H91/43/v (1494):

The luminous body will appear larger and more luminous to two eyes than to one.

That luminous body of a long shape will show itself as rounder as it is further from the eye.

On W19030v (K/P 72v, 1489-1510) he again cites both the demonstration of bringing a light closer to the eye and the sudden exposure of an eye to daylight (see below p. ): “that part of the black appears darker which is closer to the white and likewise /that part/ will appear less dark which is further from this white.” On CU459 (TPL491, c. 1492) he expresses this idea as a rule:

Precept C

On CA397rb (1497-1499) he begins to formulate a note: "Who looks at the black object on a white background" and then stops short. In the period 1505-1510 he considers these questions afresh on CU151 (TPL204) headed:

He considers this intensifying effect of contrasting colours again on CU154 (TPL231):

On the nature of the colours of backgrounds on which white borders.

On CU184 (TPL238c, 1505-1510) he gives another example of the effects of contrasting colours in a passage entitled:

On the nature of comparisons

He therefore urges that contrasting backgrounds are more appropriate, as on CU148 (TPL229, 1505-1510):

Of the backgrounds that are more appropriate for shadows and lights.

While he generally recommends that white should always border on dark and conversely, on CU150 (TPL230, 1505-1510) he considers a situation where this is not the case:

How one should act when white terminates on white or dark on dark.

On TPL206 (1505-1510) he suggests that a colour seen against a background of the same colour appears more beautiful:

What part of a same colour will show itself as more beautiful in /a/ painting

This idea he restates on TPl217c (1505-1510):

What part of the surface of bodies will show itself of a more beautiful colour?

Meanwhile, his experiments with camera obscuras had made him aware that the usual rules of contrast do not always hold. On CA195va (1508-1510) for example, he explains:

Nonetheless such cases remain the exception and he continues to favour situations where contrasting colours are positioned opposite one another. On CU183 (TPL190a, 1505-1510), for instance, he expresses this in terms of a rule:

A second rule follows:

He restates these precepts on CU145 (TPL232, 1505-1510):

On the boundaries of objects

On CU153 (TPL252, 1508-1510), he returns to these ideas:

Backgrounds

He reformulates this on CU751 (TPL769, 1508-1510):

In the late period he returns to this theme on CA184vc (1516-1517) in a passage entitled:

Of colours

Other passages concerning this theme have been cited elsewhere (see vol. one, part three.1).

7.2.iv. Sudden Exposure to Darkness

In addition he considers a converse demonstration, namely, when the eye suddenly leaves the daylight in order to enter an enclosed space, as, for example, on Forst II, 2 158v (c. 1495) under the heading:

Of perspective.

At this point he introduces and answers a potential objection:

This objection answered, he continues with his train of thought:

On L41v (1497, 1502-1503) he describes how a person sees a dark place from the outside (see below p. ):

On Painting.

He then mentions what happens when this person enters a dark place:

He develops this description of a person entering a dark place on Mad II 70v (1503-1505), opening with a general statement and a question:

The pupil of the eye expands and contracts with slowness in seeing dark or luminous things.

To which he replies:

He returns to this demonstration once more on D7v (1508):

7.2.v Peashooter

He mentions a fifth demonstration to confirm the link between pupil size and apparent size on L14r-13r (fig. 1050, 1497, 1502-1503):

The test here described relates in turn to his subsequent experiments concerning the role of backgrounds in perception (see below p. ).

7.3 Theoretical demonstrations

In addition to these five practical experiments he also pursues the connection between pupil size and apparent size in more theoretical terms as, for instance on D7r (1508) under the heading (fig. 1051):

How a larger pupil sees a larger object.

On D4r (1508) he draws the pupil in relation to the cornea in the form of a cross-section (fig. 1052) accompanying which he offers a further theoretical demonstration:

7.4 Madrid chapter

In the Madrid Codex he develops his ideas on pupils in a series of paragraphs which amount to a chapter on the problem. This opens, on Mad II 25r, with a discussion of pupils of different shapes:

A paragraph devoted to the case of cats follows:

He then turns to the human eye: “The eye of a man placed in the great light of the air or some other luminous body contracts the pupils and makes it of lesser power whence places of lesser light will appear tenebrous to it.” On Mad II 25v he compared the amount of light seen by one and two eyes respectively (see below p. ), which leads to a comparison between small and large light sources:

No place in nocturnal times, is totally deprived of light, but it appears dark to diurnel animals.

In the final paragraph he returns to the theme of changing pupil size: “Every pupil continually changes in diverse sizes, in diverse qualities of light or darkness, but above all the others, the owl makes the greatest change.” In the right-hand margin he illustrates these changes in size of owls' pupils. In a first diagram he draws (fig. 1053) two tiny pupils marked b and c, connected with a large circle a which he describes in the caption: "a is the imprensiva in seeing luminous objects." Directly below he writes: "a is the imprensiva of the owl."

This is followed by a second diagram (fig. 1054) in which the imprensiva a is dwarfed in comparison with the enormous size of the pupils in which are inscribed the words "nocturnal pupil." Finally, he makes a comparative diagram (fig. 1055) in which each of the pupils is drawn as three successively larger circles, marked a, b and c with the caption alongside: "a is the light of the sun; b is the daylight; c is the nocturnal light that the long-eared owl receives." Beneath this diagram is a further marginal note on human eyes: “ Men overcome by excess light practically close the eyes, cutting off part of the pupil with the lids of the eyelids of the eyes, because by this means it is diminished as much as it can be.” On Mad II 26r there are notes on painting. On 26v he returns to the connection between pupil size and light intensity, under the heading:

Of perspective.

He interjects a note why this is indirectly important for painting: “Even though this case is not to be enumerated among the precepts of painting, I nonetheless do not wish to exclude it because it is necessary for speculators.” A general claim follows:

By way of demonstration he cites his experiment of looking at a star through an aperture.

Immediately following he offers a second demonstration involving the candle experiment:

At the bottom of Mad II 26v he outlines a third experiment:

On Mad II 27r, the folio opposite, he gives a more detailed account of his candle demonstration:

He now returns to the case of pupils in nocturnal animals:

Some pupils of owls are, in turn, compared with those of men:

In the margin he pursues this comparison between size of pupil and imprensiva:

Beneath this he draws a rough sketch of a small pupil and large imprensiva (fig. 1056) and with this his excursus on pupils ends.

7.5 Comparative Studies of Pupils in Man and Animals

The comparisons between pupils in owls and men in the Madrid passages just cited reflect a theme that fascinates Leonardo. Perhaps the earliest extant note on the problem is on H86/38/r (1494):

On CA262rd (1506-1508), on a folio where he also discusses camera abscuras (see above p. ), he begins with variations in the human pupil:

If the...darkness of the night is 100 degrees of darkness than in the evening and the eye of a man doubles /the size/ of its pupil, this darkness diminishes by half to this eye because, having doubled half of its visual power, there now remain for it 50 degrees of obscurity of darkness.

This he compares with the case of an owl:

figure)

An objection is now raised and dismissed:

In the right-hand margin he draws both a small pupil of a man and the potentially large pupil of an owl (fig. 1059) which he identifies and beneath which he adds a caption:

The pupil c is its size during the daytime, that is, in the greatest brightness of the day.

In the second portion of the text on CA262rd he considers the pupils of the leonine species and problems of pupils of different shapes:

In the margin he draws a slit pupil in the shape of a thin rectangle (fig. 1058). In the main text he then considers the consequences of such a pupil for the visual process:

To illustrate this he draws a small marginal diagram (fig. 1062) followed by a larger one which intrudes on the text space (fig. 1063). Around this he explains:

In this context the reason for his studies with slit-shaped camera obscuras becomes clear (see above p. ). Despite all these studies he remains unsatisfied. On CA360RC (1508-1510), for instance, he asks anew: "Why the pupil becomes smaller the more light it has in front of it and similarly, in the opposite /situation/, expands in the dark?". On W19042r (K/P 42r , 1489-1510), he offers an answer, beginning with two claims about nocturnal animals:

This leads to comments concerning images being "all in all the eye."

The visual power is all in all the pupil and all in every part.

It follows that half of the pupil sees the object integrally as if it were entire.

He now returns to his connection between pupil size and the apparent size of objects:

In his final example on this folio he considers an experiment comparing effects of monocular and binocular vision (see below p. ). He pursues his comparative studies of senses in man and animals on W19030v (K/P 72v, 1489-1510):

A comparison of eyes in lions and men follows:

This leads to a comparison of their pupils:

On D5v he continues this discussion, beginning with two chapter headings in the margin: “Why nocturnal animals see more by night than by day? On the eye of man in comparison with his brain.” Opposite this is an introductory paragraph:

The folio ends with the instruction "turn the page" to 5r. Here the comparison between man and nocturnal animals is reformulated:

A comparison of imprensivas follows:

This discussion is again summarized by two marginal notes:

On BM Arundel 64v (1509) he explores how a bird's eye and pupil opens and closes (fig. 1060):

It closes from below because it descends from above.

He takes up the theme of comparative pupil sizes anew on G44r (c. 1510-1515) in a passage headed:

On the eyes of animals.

In the margin he draws (fig. 1061) two sets of three pupils marked omn and caa respectivally, the first set representing the pupils of owls, the latter, pupils in the leonine species. Above these he writes a marginal note:

In the main text he compares these with human pupils:

Having reached the end of the folio he continues in the lower half of the right-hand margin: “they are constrained to go out into the illuminated air of the sun and they diminish the pupil by so much that the visual power diminishes together with the quantity of such light.” A late diagram on CA243va (c. 1513) which, as Pedretti has s hown, is connected with W12443r (figs. 1068-1069) further illustrates connections between the pupil and camera oscura (cf. figs. 1064-1067).

7.6 Towards Quantification

Throughout the above mentioned notes Leonardo is tending towards a quantitative approach to the problem. Hence on Mad II 25 he alludes to how a man's pupil expands twofold at night, whereas, in a long-eared owl, the pupil expands one hundredfold. On CA262rd he also uses these ratios, once more on W19030v (K/P 72v, 1489-1510) and again, in modified form, on D5r (1508) where he claims that a man's pupil expands twofold but its power fourfold, whereas the pupil of a long-eared owl expands tenfold and its power hundredfold. On D5r his love of successive ratios leads him astray. He claims, for instance, that the imprensiva is 10 times the size of the eye and the eye, in turn 100 times the size of the pupil, which would mean that the imprensiva is 10,000 times larger than the pupil: clearly an exaggeration.

Leonardo's problem is that although his thoughts are coherent, he is disorganized in presenting them. This weakness prevents him from retrieving efficiently what he has already done and building on his earlier efforts. As a result the quantitative urge comes and goes without resulting in ever more accurate observations and clearly organized tables.

7.7 Rules

In the end he contents himself with a series of general rules concerning the relations of pupil size, apparent size and apaprent brightness. The earliest hint of such a rule can be traced back to Forst III 36r (1490-1493) under the heading:

What thing is better seen.

This rule concerning size he expresses more clearly on Mad II 127v (1503-1505):

On painting, that is, on the eye.

At this point he is still apologetic concerning its relevance for painting and adds (see above p. ):

By 1508, on F37r, he carefully numbers his rules:

By 1513-1514, on E17v, these ideas emerge in a series of six propositions under the heading:

Painting.

Sixth: But all colours positioned in luminous places will never vary from their essence.

His pupil studies had begun as a strictly optical problem. In the late period they have become integrated within his concept of painting.

8. Uvea

In terms of modern anatomy the uvea or grape-like tunic is the pars iridics retinea directly behind the iris. Leonardo's conception of the uvea is different, namely, as a reflecting sphere surrounding the vitreous body - which he calls the albugineous humour - and concentric with the crystalline sphere, (D7v, 1508). He mentions the term uvea at least 16 times and illustrates it no less than seven times (figs. 1072-1076, Chart 24). From these diagrams it is clear that the position of the uvea in his interpretation corresponds roughly to the actual site of the retina.

8.1 Function of the Uvea

In various diagrams he depicts the uvea as if it were a mirror which serves to reflect rays either towards the crystalline sphere, as on Forst III 24r (figs. 1070-1071, c. 1490-1493), Forst III 56r (fig. 1072, 1490-1493) and K119/39/ (fig. 1079, 1504-1509) or directly towards the optic nerves, as on D10r (fig. 1076, 1508). He is interested in the possibility that the uvea may be more than merely a mirror. On DA190vb (1505-1508), for instance, he asks, "whether the image (eidola) is in the concavity of the uvea or truly in the crystalline sphere and not in the uvea." On this same folio he records his intention to "make such an eye and of glass as a natural one" (fig. 1074) and makes another drawing (fig. 1075) in which he notes that "a sees the surface of its sphere nSt tinged by the darkness of the uvea."

On D7v (figs. 1080-1081) he explores further the role played by the uvea while discussing the possibility that the visual power is situated in the crystalline humour:

On D10r (1508) he explains why he rejected the possibility that the uvea might be the seat of the visual power, under the heading:

Doubt concerning the impression of the images (eidola) in the eye.

This leads to his own theory concerning what actually happens in the eye:

Ultimately the uvea may reflect images but that is all.

9. Crystalline Humour

He refers to the crystalline humour at least 6 times and to the crystalline sphere no less than 16 times. In his early notes he tends to equate the crystalline humour with the aqueous humour. This he suggests on CA85va (1503-1504) and illustrates on CA204rb (1490-1492) where he points out that the crystalline humour occupies the "quarter bef" (fig. ). Similarly on A78 (1492) he refers to the "subtle humour called crystalline that stands in the cornea (luce). Later he conceives of the crystalline lens as a sphere at the centre of both the vitreous body and the eye. From Galen¹⁰ through to Pecham¹¹, various authors (cf. figs. 957-962) had situated the crystalline lens at the centre of the eye. Leonardo clearly intended to study the rpecise position of the crystalline more carefully. On CA345vb (c. 1508), for instance, he notes:

In the extant notes there is no record of his having carried out this plan. He does, however, provide a series of illustrations showing the crystalline sphere in the centre of the eye on K/P 118v (W19150v, fig. , 1508-1510); Manuscript D2v, 3v, 7v, 8r and 10r (figs. , 1508). On D8r he begins writing "crystalline," crosses this out and replaces it with "vitreous." On both D8r and 3v crystalline and vitreous are synonymous.

9.1 Function of the Crystalline Humour

He assumes that the crystalline lens is like a sphere of water which refracts images at its surface and then inverts them. He describes the general properties of such spheres of water and gives instructions concerning their construction. There is no record of his making quantitative records of the refractive properties of such spheres of water (see below pp. ). In a majority of cases (figs. ) he assumes that an inversion of images occurs at the centre of the crystalline sphere. In Manuscript D he explores alternatives. On D3r, for instance, he draws (fig. ) two rays being refracted at the front of the crystalline sphere, passing through it along parallel lines and being refracted inwards on re-entering the vitreous humour. On D3r, he also considers the possibility that images could pass through the crystalline without any refraction. On four other occasions, D3v, 8r, 8v and 10v (figs. ) he considers an alternative that the crystalline sphere might refract images at both its front and back surfaces without actually inverting them. On each occasion he represents this refraction in slightly different terms. On D5r he also explains why he believes that the crystalline lens is capable of varying its density:

10. Vitreous Humour or Body

In the early extant notes he has no term for the vitreous humour. In the Manuscript D (1508) he refers at least three times to an albugineous sphere and no less than 8 times to an albugineous humour. In most cases (figs. ) this albugineous humour is synonymous with what would today be termed the vitreous humour or vitreous body. In the Manuscript D he also refers at least once to an omore vitrea and no less than 15 times to a spera vitrea. On D8r, he begins by writing "crystalline," crosses this out and writes "vitrea." Diagrams on D8r, 3r, and 3v (figs. ) confirm that, in his mind, spera vitrea, is synonymous with crystalline sphere and not the vitreous humour in the modern sense.

10.1 Function

In the early notes the role played by the vitreous humour in the visual process is neither discussed nor illustrated. In the Manuscript D he shows (figs. ) rays passing directly from the aqueous humour in the cornea, through the pupil and into the vitreous humour without any refraction. On D3v, 8r, 8v and 10v (figs. ) he draws the rays as being refracted in passing from the vitreous to the crystalline, then refracted anew in passing from the crystalline back into the vitreous humour and intersecting a second time in the vitreous humour before entering the optic nerve. Hence the vitreous humour becomes the place where images are re-inverted in order to reach the brain right side up.

11. Retina

In Leonardo's models of the eye the uvea takes the place of the retina. But its function remains very different. The uvea as he conceives it merely reflects images like a mirror: it does not record images in the way the retina is now known to do.

12. Orbit

He refers to the orbit or eye-socket as the case of the eye (cassa dell'occhio) on at least three occasions. In addition he draws it both frontally and from the side a number of times (figs. 1082-1091, Chart 24). On W19058v (K/P 42v, 1489-1510) he includes both the superior and inferior orbital fissures and indicates the superorbital and infraorbital foramina (fig. 1083). He does not discuss the orbit's function.

13. Optic Foramen

On W19059v (K/P 40r, 1489-1510) he refers to the optic foramen as the "aperture of the case of the eye" (buso della chassa dell'ochio). Although he has no technical term for the foramen, he shows it clearly in a number of his anatomical drawings (figs. 1092, 1093, Chart 24).

14. Second or Optic Nerve

In his extant notes he refers to optic nerves at least 21 times (Chart 24). A majority of these obviously refer to the second nerve. In his drawings he indicates the path of this second or optic nerve at least 11 times (e.g. figs. 1113-1119). On W19057r (IK/P 43r, 1489-1510), he shows the optic nerves (fig. 1092) protruding from the optic foramina. The precise origin of the optic nerve remains a problem for him. In his early drawings on W12627r, 12626r (K/P 4r, 6r, figs. 1100, 1101, 1485-1487), and BM711v (fig. , 1492) he represents this nerve as originating directly behind the pupil. Later, probably as a result of having studied Pecham¹² (fig. ), he shows it as beginning behind the crystalline sphere (figs. 1104-1105). By 1508 he tends to show the optic nerve as originating directly what we would term the retina (figs. 1117-1119).

14.2 Function

He makes some comparative studies of optic nerves. On W19030v (K/P 72v, 1489-1510), for instance, he notes that in the leonine species "the optic nerves are immediately joined to the brain" and he contrasts these with man whose:

On W19052r (K/P 55r, 1489-1510) he notes that the function of the optic nerves is to serve the visual power. On D3v, 7v and 8r (1508) he locates the visual power at the entrance of the optic nerves. Precisely how images are conveyed through the optic nerves to the senso commune and the imprensiva he does not explain.

15. Optic Chiasma

He does not have a specific term to describe the optic chiasma. Four early drawings (figs. 1082, 1100-1102) confirm that he was initially unaware of its existence. Following his anatomical studies in the period 1506-1508, he indicates the position of the optic chiasma clearlty (figs. 1104-1105, 1113-1119). In the extant notes he makes no conjectures concerning its function.

16. Optic Tract

He also has no term for the optic tract, but in the period after 1505 he draws it on at least nine occasions (figs. 1104-1105, 1113-1119).

17. Third or Motor Oculi Nerve

His interest in the third or motor oculi nerve is first recorded in a note on W19059v (K/P 40v, 1489-1510):

Of closing the eyelids

Of raising the eyelids

Of lowering the eyelids

Of shutting the eyes

Of opening the eyes.

On W19052r (K/P 55r, 1489-1510) he draws this nerve, (also called the oculomotor nerve), clearly (figs. 1104-1105). On W19116-7r (K/P 115r, 1508-1510) he makes a further note:

18. Ophthalmic Nerve or First Division of the Fifth or Trifacial Nerve (n. trigeminus)

He has no specific term for this nerve, also known today as the ophthalmic division of the trigeminal nerve, but, nonetheless, draws it on both the Weimar sheet (fig. 1117) and W19052r (K/P 55r, figs. 1104-1105, 1489-1510). In the extant notes he does not discuss the function of this nerve.

19. Ventricles

Mediaeval authors such as Albertus Magnus assumed that the ventricles were circular cavities¹³ (fig. 1097), Gregor Reisch also assumed this in his Margarita philosophica¹⁴ (fig. 1098). Leonardo's early drawings (figs. 1099-1102) stand clearly within this tradition. Sometime between 1506 and 1508, however, he injects wax into the ventricles and thus arrives at a much more accurate impression of their shape (figs. 1106-1110). He only rarely uses the general term ventricle (e.g. D5r, 1508), but has his own terms for the lateral, third and fourth ventricles respectively.

19.1 Lateral Ventricle

In an early drawing on W12626r (K/P 6r, fig. 1101, 1485-1487) he indicates the lateral ventricle as seat of both the intellect and the imprensiva. For him the imprensiva is where the images are impressed. He uses the term imprensiva at least 43 times and illustrates it 9 times (e.g. figs. 1109-1112, Chart 25). He does not discuss the function of the imprensiva in detail. Nonetheless, on Mad II 24r and 25v, he notes that two eyes affect an imprensiva twice as much as does one eye, and one Mad II 27r he considers the effects of relative sizes of imprensiva and pupil, claiming that the imprensiva in man is 40 times the size of an owl's imprensiva, (see above p. ). These comparisons he pursues on D5r (1508) where he claims that the imprensiva in man "is more than ten times the size of the entire eye of man and the pupil in which sight has its origin is less than a thousandth part of the eye, whereas the pupil of the long-eared owl, at night, is considerably larger than the ventricle of the imprensiva situated in its brain."

19.2 Third Ventricle

On W12626r (K/P 6r, 1485-1487) he shows the third ventricle as seat of both the will (volunta') and the senso comune. He uses the term senso comune at least 24 times (Chart 25). In his anatomical studies he sets out to define precisely where the senso comune is situated. On W19058r (K/P 42r, fig. 1093, 1489-1510), for instance, he notes:

On W19057r (K/P 43r, fig. 1094, 1489-1510), he locates the senso comune clearly with a drawing and the caption: "where the line am is intersected by the line cb there the meeting of all the senses is made."

19.2.i. Function

On CA90rb (c. 1490) he discusses the function of the senso comune and its relation to both the imprensiva and the memory:

He pursues this discussion on W19019r (K/P 39r, 1489-1510) under the heading:

How the 5 senses are functions of the soul.

From these passages it is clear that the third ventricle, which Leonardo terms the senso comune is also the seat of the judgment. At an earlier date he had been non-committal concerning the precise location of the judgment. On BM220v (1500-1505), for instance, he had noted:

Among Mediaeval writers such as Nemesius and Macrobius there had developed a view that the senses are reliable¹⁵, and that only the judgment was deceptive. This attitude also emerges in Leonardo's later notes. On TPL65a (1505-1510), for example, he points out that: “There is nothing which deceives us more than our judgment either in giving a bad opinion of our works or in judging as good the things of our enemies or our friends.” On CA154rb (1508-1510, cf. 154rc) he openly contrasts the reliability of of experience, i.e. the senses, with the deceptiveness of judgment:

On CA29va (1509-1510) he goes on to compare the deceptions of judgment with deceptions of vision:

19.3 Fourth Ventricle

On at least 6 occasions he designates the fourth ventricle as the seat of memory (Chart 25, e.g. figs. 1110, 1117).

20. Visual Power

Leonardo mentions the term visual power (virtu visiva) at least 66 times and an alternate form, (potentia visiva) no less than four times. Its precise nature at first remains ambiguous: is it active int he manner of the ancient theory¹⁶ which held something active enamates from the eye or is it passive like a screen or film which records images? At the outset in 1492 Leonardo assumes that the visual power is situated in a point on or within the pupil. Various experiments (see above ) persuade him that this power is not only in one point but "all in all and all in every part" of the pupil. At the same time, his general conception of physics leads him, on D1r, to argue:

Why Nature did not make the visual power equal in power in all its parts.

Meanwhile, in the period 1505-1508 he explores further alternatives: that the visual power might be situated in the crystalline sphere, in the uvea or at the beginning of the second or optic nerve (Chart 25). He decides finally that it must be situated at the beginning of the optic nerve, and that the visual power is not something which emanates from the eye, but rather something which passively waits for images to come to it. Precisely how it conveys the information of those images to the brain, he does not explain.

21. Conclusions

With respect to the eye Leonardo wishes to reject the authority of previous authors and base his claims strictly on experience (CA119va). This is not to say, however, that he ignores earlier sources entirely. We know that he studied Pecham's Optics directly (see above p. ) and hence it can hardly be a coincidence that there are very close parallels between Pecham's notions that the optic nerve protrudes into the crystalline lens and Leonardo's opinions. A detailed analysis of each part of the visual process discussed or illustrated by Leonardo, reveals how his attention paid to different parts varied enormously (see Chart 20). In rare cases such as the "orbit" or "optic chiasma" he has no term for a part although he illustrates them beautifully. In most cases he mentions a term less than 25 times. He uses the term senso comune 25 times, imprensiva, 43 times; and both virtu visiva and luce 70 times. By far, his most frequently used term is popilla (253 times).

It was noted that the meaning of both luce and popilla remain ambiguous. Both terms embrace a series of connotations which range from pupil, through cornea to the eye in general. The reason for their surprising importance in the notebooks is because Leonardo believes that variations in pupil size play a key role in all estimates of apparent size. Although Leonardo pleas for an experimental approach to vision, it is important to note that he is only able to carry this out in stages. By 1490 he has analysed the orbit and optic foramen. (This provided no difficulty because skulls were relatively easy to come by.) In the period 1506-1508 he makes his experimental demonstrations concerning the optic chiasma and optic nerves, as well as the ventricles.

By 1504-1509, he also interested in dissecting the interior of the eyeball itself. But this he appears never to have accomplished. Was it from lack of time? Or was he not allowed? We know that he had trouble with permission to do anatomy in the late period (CA182v). One thing is fairly certain. In the early period, when there may have been more opportunity for anatomical study, Leonardo had little interest in the structure of the inner eye. Only gradually did he recognize that the nature of the visual process behind the pupil was a problem. And it may well be that by the time he saw the problem fully, the opportunities for studying it were no longer fully there. How he came to see that the visual process was a problem is the subject of the next chapter.

Notes

1. Aristotle ↩︎
2. Cicero ↩︎
3. Aristotle ↩︎
4. Cicero ↩︎
5. Leonardo ↩︎
6. Witelo ↩︎
7. Missing Citation ↩︎
8. Leonardo ↩︎
9. Leonardo ↩︎
10. Galen ↩︎
11. Pecham ↩︎
12. Pecham ↩︎
13. Albertus Magnus ↩︎
14. Gregor Reisch - Margarita philosophica ↩︎
15. Nemesius and Macrobius ↩︎
16. Leonardo ↩︎