[In the following essay, Gombrich calls for a new edition of Leonardo's Trattato della Pittura, one in which the problems of the “derivation and date” of particular items are addressed, and one which provides an analysis of the relationship between Leonardo's theories and practice.]

In accordance with traditional usage I mean by Leonardo's Trattato della Pittura the collection of the master's notes compiled by Melzi and preserved for us in the Codex Urbinas 1270 (henceforth abbreviated CU), of which a facsimile edition together with an annotated translation with numbered paragraphs has been published by A. Philip McMahon (Princeton, 1956), (henceforth abbreviated TP McM).¹ Despite its fragmentary nature and somewhat confusing arrangement it represents the outline of a project of awe-inspiring ambition, a project such as only Leonardo could have conceived but which even he could never have completed—the plan to record in words and diagrams all recurrent phenomena of the visible world which are the painter's legitimate province.

Naturally this gigantic enterprise has never been neglected by students of Leonardo, and yet it may be said that his individual propositions and observations in this field have been less fully analysed than has his corpus of anatomical studies or his mechanical investigations. I shall argue here that a new edition of the Trattato in the light of these and similar precedents is one of our desiderata, and that such an edition would have to address itself not only to the problems of the derivation and date of individual items (admirably studied by Carlo Pedretti²), but also to the relation of Leonardo's theories to his practice, to the validity and purpose of this individual statements, and to the relation of his conceptions to the traditions he inherited. Such an edition would probably demand the collaboration of many scientists, geologists, botanists, and psychologists specializing in what J. J. Gibson has called “The ecological approach to visual perception”³—a phrase admirably describing one of Leonardo's purposes. All that is intended here—as indicated in my title—is to draw attention to “some questions and desiderata”.

I hope I may be forgiven if I approach these questions from the point of view presented in my book on Art and Illusion.⁴ I tried there to sum up the thesis of that book on the psychology of representation in two connected formulas: the first that ‘making comes before matching’ and the second that what we call the faithful imitation of nature can therefore only be achieved through the process psychologists describe as ‘schema and correction’.

Speaking in these categories Leonardo must be seen essentially as a ‘maker’ rather than an imitator. I has always seemed to me that this celebration of the painter's creative powers in the Paragone (TP McM 35, CU 5) represents much more than a magnificent piece of rhetoric. It adumbrates the programme the master was to follow all his life. He wanted to be and he was a creator of people, scenes, events, indeed of new worlds through his knowledge of the forces that governed the laws of the universe. But it may be argued that to carry out this program he had to practise an unusual economy of means. However much he celebrated the powers of the human eye it was not from visual observation that he derived the basic elements of this creation, but from two contrasting sources—tradition and reasoning. It should not demand any detailed demonstration to show that many of the basic schemata which Leonardo used throughout his creative life were those he had learned in the workshop of Verrocchio. This has been frequently observed in relation to his favourite physiognomic types, the man with the ‘nutcracker profile’ and the female head with the enchanting smile.⁵ It is true that Leonardo also seemed sometimes to struggle against the compulsion to repeat these schemata, but when he did so he mostly called in aid the power of reasoning rather than of observation. I have attempted to connect his socalled caricatures with this effort to vary the schema by means of systematic variations and permutations⁶ but it still remains in evidence—to use a scholastic term—as forma formans trans formam formatam translucens.

The tenacity with which the master stuck to inherited schemata in his landscape seems to me even more astonishing. Nothing could strike us as more creative and dramatic than the landscape backgrounds of the Mona Lisa and the Louvre St. Anne.⁷ One is again reminded of his exaltation of the painter's power in the passage from the Paragone invoked above: “… if he wishes from the high crests of mountains to disclose a great countryside and if, after that, he wishes to see the horizon of the sea, he is their lord, and so too if he wishes to see high mountains from deep valleys, or from high mountains to see low valleys and seashores” (TP McM 35, CU 5). But it was in his mind's eyes that he perceived the panoramic visions of infinite expansion and complexity he painted, for no earthly eye has ever seen such formations.

This is not the place to review once more the role of landscape motifs Leonardo has left us, but it seems to me that the topographical element in many of these works, or at least their topographical fidelity has been overrated by many commentators.⁸ The elements out of which Leonardo created his landscapes also derive from the tradition he had absorbed in his youth. Like the stepped rocks I discussed elsewhere⁹ the formulas for mountains in the shape of steep cliffs can also be traced in an unbroken tradition from Hellenistic painting (Odyssey landscapes) via Byzantine conventions to the stage props used by Tuscan painters where the ‘schema’ is slowly ‘corrected’ so as to approximate a plausible reality. Even so, when Leonardo in his early sketch for a Madonna and Child (W 12276) wished to indicate a landscape background he rapidly drew a steep triangle to stand for a mountain. He continued to use the schema in the background of many of his paintings such as the Uffizi Adoration and the St. Jerome, but he also knew how to transform and transfigure the motif throughout his artistic career. From the Paris version of the Madonna of the Rocks to his fantastic drawings we can observe how he modified the elements through changes in structure, in the fall of light and through the effects of atmosphere. Like a musician who choses a simple theme in order to display the power of his inventive imagination, Leonardo placed his stage props into the gloom of caverns, in sunlit planes, on distant horizons or in thunderclouds to show how the schema was metamorphosed through what he might have called the ‘accidents’ of illumination and distance. This is not to deny that he occasionally incorporated in his compositions elements he had observed and drawn during his explorations of Alpine scenery, but the selection of these motifs was again guided by his visual vocabulary. Indeed it might even be argued that his geological theories were not uninfluenced by that schema he had inherited. It is well known that he considered the shape of peaks and ranges to be entirely due to the action of water.¹⁰

Hence to him the steep precipices where the rock appears to have been cut as with a knife struck him as most characteristic. The shape of ‘folds’ which influence our idea of mountains had no place in his thoughts. As a maker rather than a portrayer of mountain scenery he considered them as little as did the ancient Chinese. Only as an engineer concerned with the lie of the land as in his studies of the Tuscan hills, made in connection with the diversion of the Arno, did he attend to these features.¹¹

I should like to ask whether the type of detailed commentary of the Trattato which seems to me desiderable would not also reveal a similar selectivity of vision, marked by the master's power of combination and permutation. I am aware of the immense variety of observations we find in these dazzling notes dealing with the phenomena of nature, but I suspect them to be more systematic, indeed a priori, than purely empirical. Admittedly this interpretation, which still remains to be tested, is at variance with the conception of Leonardo's procedure put forward by such authorities as Ludwig Heydenreich to whom we owe the perceptive introduction to McMahon's standard edition of the Trattato. My reservations about his conceptions go back over many decades. In fact I remember a conversation I had with him some thirty years ago in a London restaurant, when he stressed the role which the eye played in Leonardo's scientific and artistic life and I was prompted to ask him whether he really thought that the view through the window resembled in any way any of Leonardo's creations? I would not have asked the same kind of question if we had been talking about Camille Pissarro. Such a question and such a comparison only appears unorthodox and almost illicit because most art historians are brought up in a relativist creed. Even if they may grant that the world looks to them more like a painting by Pissarro than like one by Leonardo, they may still think it possible that Leonardo and the Impressionists “saw” the world exactly as they represented it. In other words they refuse to commit themselves to any objective standards by which we can discuss the transition from making to matching or from the schema to its correction. The kind of commentary I should like to see of Leonardo's propositions in the Trattato would be stillborn if it is assumed that he must have seen what he described.

Anatomists, to repeat, do not take that view. They allow us to stand in awe before his drawings and descriptions of the muscles and organs of the human body while pointing out his debt to the tradition and the occasional errors he incorporated in his creations. They know, as he also knew, that science is not simply based on looking but on informed questioning. But while Leonardo the anatomist had a whole corpus of knowledge at his disposal which he could probe and correct, the task of sistematising the visible phenomena of our experience had lamost been confined to the study of optics. Considering the narrow base on which he had to build his edifice of observation his success is indeed astounding, however puzzling some of his remarks may be.

I should like to turn to some examples of such puzzling features as they can be found in Part V of the Trattato the section entitled, “Of the shadowed and bright parts of mountains” (TP McM p. 272 note, CU 231) though the scribe found he had inserted the heading prematurely and only came to the point on the next page (TP McM 824, CU 231 v) Fig. 1:

Of the peaks of mountains as seen from above. The peaks of mountains, seen one beyond the other from above, do not grow bright in proportion to the distances of those mountain peaks from one another, but do so much less, according to the seventh proposition of the fourth book, in which it is stated: the distances of landscapes seen from above grow darker toward the horizon, and those which are seen at the same distance as the first but from below always/grow lighter.

This comes about according to the third proposition of the ninth book in which it is stated: the thickness of the air seen from below is much brighter and more resplendent than that seen from above, and this happens, because the air seen from above is somewhat penetrated by the dark visual images of the earth which is below them, and so it appears to the eye to be darker than that which is seen from below, for that air is penetrated by rays of the sun that reach the eye with great brilliance.

Thus the same thing happens with mountains and the landscapes in front of them, the visual images of which, passing through the air, appear dark or bright, depending on the darkness or brightness of the air.

The structure of this statement is the same as that of many others in the Trattato: a visual phenomen is described and then explained by reference to general propositions which are cited by ‘book’ and ‘number’. It would be the task of the commentary here envisaged to extract and explain all these propositions in the light of the theories applied by Leonardo, for instance the idea derived from Epicurean optics that the darkness mentioned is due to “the visual images which fill the air from below”. But the commentary would also have to address itself to the question as to the nature of the observations on which Leonardo here relies. Do landscapes seen from above generally grow darker towards the horizon?

Used as many of us are to look down at a landscape from the window of a plane we do not have to wait for such exceptional opportunities as Leonardo may have enjoyed when he climbed the Monte Rosa.¹² Whatever he may have seen on that particular day, we will not find it to be the norm that landscapes grow darker towards the horizon. But was it really an abservation which led Leonardo to pronounce this rule or was it again a memory of what he had learned as a painter's apprentice? Cenino Cennini¹³ codifies Tuscan Trecento practice as follows:

On the way to paint a mountain, in fresco or in secco. chapter LXXXV. If you want to do mountains in fresco or in secco, make a verdaccio color, one part of black, the two parts of ocher. Step up the colors, for fresco, with lime white and without tempera; and for secco, with white lead and with tempera. And apply to them the same system of shadow and relief that you apply to a figure. And the farther away you have to make the mountains look, the darker you make your colors; and the nearer you are making them seem, the lighter you make the colors.

What may have led to this rule of thumb is the fact that tonal contrasts diminish with distance. This fact which was known to the ancients and became incorporated in medieval practice¹⁴ leads to the avoidance of strong colours in the distance and a general veiling of tones which might well be confused with an over-all darkening. A glance at Leonardo's own practice, notably the backgrounds of the Mona Lisa and the Louvre St. Anne convinces us that he fully mastered the skill of manipulating contrasts, stepping them down to achieve the impression of distance with truly compelling effect.

Moreover Leonardo seems to pass over the rule he stated in the previous note according to which the mountain peaks can look bright or dark from different standpoints. In any case he insists in a number of notes and diagrams that the peaks are always darker than their base. Did he simply generalise here on an observation he made in the Po Valley when the hazy mists might have caused the foothills to look whiter than the soaring heights? Certainly this contrast is not the norm, nor did Leonardo treat it as such in his paintings and drawings,¹⁵ but the Trattato has it otherwise; as in TP McM 826, CU 232 v (Fig. 2).

Why distant mountains appear/darker at the summit than at the base. With regards to that which has already been said on the preceding page, I continue by saying that although the spaces between the mountains, AO, OP, and PQ, are equal in proportion, the summits of these mountains, O, P, Q, do not observe the same proportion in respect to their brightness, as they would if they were of the same height, because, if they were of the same height, their extremities would be in air of equal density, and then the proportions of distances and of colors would be the same. But such an arrangement cannot be shown to the eye because, if the eye is as high as the summit of the mountain, it is necessary that the summits of all the mountains that are beyond the first one should be at the height of the eye and of the first mountain, and from this it follows that the second mountain and the third, and also the others that follow, would not exceed nor be exceeded in height by the first mountain, nor by the eye. Therefore, as the surface of the summit of the first mountain is even with the summits of all the mountains that are behind the first, these cannot be seen, except for the summit of the first one. Thus this demonstration is vain, for example: let A be the eye, B the summit of the first mountain, C, D, the other summits; and you see how the summit B/ meets (the height of) the other two summits, C and D, and the eye A sees the three summits B, C, D, within the outlines of the mountain B. And the distances and colors of these are similar in proportion, but neither the distance nor the color is seen.

The paragraph with its three lettered diagrams is of some interest in my context, because it shows Leonardo's a priori reasoning at its extreme. Far from being concerned with what the eye sees, it demonstrates at some lenght what the eye cannot see, or as he himself puts it, that “this demonstration is vain”. We obviously cannot see mountain peaks hidden behind peaks of the same height if our eye is level with them, and therefore we cannot tell how dark their summits would look if we saw them.

But this, we may supplement the argument, is a limiting case, which seems only to have been discussed for the sake of completeness. The next section (TP McM 823, CU 233, Fig. 3) discusses the case when the mountains of equal height are seen from a higher station point. In that case, of course, we must take account of their distance from the eye rather than from each other and, again, of the varying thickness of the air through which they are seen. It is here, again, that the next paragraph (TP McM 833, CU 233 v, Fig. 4) reverts to the thesis that the higher peaks must look darker. The reasoning is reiterated in TP McM 827, CU 234 (Fig. 5), with more diagrams and yet again in the subsequent paragraph, TP McM 828, CU 234 v (Fig. 6). There is yet another diagram and brief restatement of that principle in TP McM 830, CU 235 v (Fig. 7).

It must be granted that some of these paragraphs may represent various versions of the same note Leonardo was trying to draft. Even so his obsession with an untypical phenomenon is intriguing. It becomes intelligible, however, if we consider his ambition to systematize the rules of aerial perspective as precisely as those of linear perspective had been systematised.

As the name implies aerial perspective could be interpreted as a consequence of the ‘thickness’ of the air which, on level ground, would affect the appearance of objects in proportion to their distance. The varying thickness of the air in the vertical dimension obviously introduced a complicating factor which Leonardo wished to incorporate in his system. He must have reasoned that if dense air tones down the colours, the thinner air in the higher regions must preserve darkness. Only after having established this general proposition did he introduce such further modifications of the schema as the variations of the weather, (TP McM 832, CU 235) the seasons (TP McM 834, CU 235) and of vegetation (TP McM 835, CU 237 v). Finally, in TP McM 839, CU 236 v/237, Leonardo the naturalist takes over from the theoretician.

The soil is drier and more meager on rocks, of which mountains are composed. The trees will be smaller and thinner the nearer they are to the summits of mountains, and the soil will be more meager the nearer it approaches those summits; and more abundant and rich the/nearer it is to the concavity of the valleys. Therefore, painter, you will show the summits of mountains composed of rocks, in large part not covered by soil, and the grass that grows there in large part turned pale and dry through lack of moisture, and the sandy and meager soil appearing among the pale grass. And show small bushes, dwarfed and aged and diminutive in size, with short, thick branches and few leaves, revealing in large part their decayed, dry roots interwoven with the cracks and broken places of the decomposed rocks, created by chips broken off by men and by winds. And as you descend farther toward the bases of the mountains, the trees are more vigorous and thick with branches and leaves, and their verdure is of as many kinds as there are species of plants that compose the forest. This spread of branches is manifested in various ways and in varying densities of branches and leaves, and also in varying shapes and heights; some trees having thick branches an leaves, such as the cypress and the like, and other with sparse and expansive branches such as the oak, the chestnut and others; some with minute leaves, and others with slender ones, such as the juniper, the plane tree and the like; some masses of trees having become separated by spaces of different sizes,/and others united, without division by meadows or other spaces.

The vivid and detailed description of the variations of vegetation encountered during a mountain tour leaves no doubt that he speaks from experience. But the question remains: would Leonardo ever have match such a word-painting in a real painting?

Part six of the Codex Urbinas may here offer more food for thought. This lengthy section on Trees and Foliage is particularly rewarding in showing the combination in Leonardo's method of a prior reasoning and detailed observation.

Trees and greenery had of course formed an element of painting since classical antiquity and there was a rich stock of schemata for the artist to apply. But in this case, it appears that Leonardo was determined to correct the basic schema from the outset, starting with the structure of trees and going on to the rendering of their foliage in the ambient air. Once more it appears that the fundamental ‘correction’ of the schema is based on reasoning rather than observation. In concerning himself with the morphology of trees he characteristically started with the laws of their growth.¹⁶ In a diagram which we have both in the original (Ms M 78 v) and in the TP McM 900, CU 246 (Fig. 8), he implicitly compared the branches of a tree with the delta of a river. Every division must diminish the timber or water available in equal proportion, so that any section through the branches at an equal distance from the trunk or river course must add up to the equal amount of matter (TP McM 899, CU 244 v and 245; TP McM 947, CU 266 v/267).

Consulting a dendrologist I found that he was much impressed by Leonardo's exercise in ‘biometrics’, though the ideal law he postulated rarely quite holds. Leonardo himself reflected on the reasons for this modification of the schema, though he explicitly says that (TP McM 900, CU 246 v, Fig. 8). “Although these things do not serve painting, yet I will note them, so as to leave out as little about trees as is possible” (a reference to the way tree rings will indicate fluctuations of weather and climate). It would be of great interest if botanists could comment on the detailed descriptions of the characteristic growth patterns of various species and on Leonardo's theories concerning the advantages the tree derives from them (TP McM 887, CU 247).

But here, as in the case of mountains, the study of structure, of morphology, is only one aspect of the painter's concern. He must know how these structures appear to the eye in the varying conditions he wants to represent them. Of course this task of representing objects not as they are but as they appear to the eye is part and parcel of the Western tradition of art. In the rendering of trees and foliage this need has always been explicity acknowledged. It is notoriously impossible to paint every twig and leaf of a tree; all the brush can render is the global impression, achieved through some kind of summary treatment. There was a convenient schema at hand for the rendering of trees and leaves which is well described in Cennine's Handbook¹⁷:

The way to paint trees and plants and foliage, in fresco and in secco. Chapter LXXXVI. If you wish to embellish these mountains with groves of trees or with plants, first lay in the trunk of the tree with pure black, tempered, for they can hardly be done in fresco; and then make a range of leaves with dark green, but using malachite, because terre-verte is not good; and see to it that you make them quite close. Then make up a green with giallorino, so that it is a little lighter, and do a smaller number of leaves, starting to go back to shape up some of the ridges. Then touch in the high lights on the ridges with straight giallorino, and you will see the reliefs of the trees and of the foliage. But before this, when you have got the trees laid in, do the base and some of the branches of the trees with black; and scatter the leaves upon them, and then the fruits; and scatter occasional flowers and little birds over the foliage.

The chasm which separates this precept from the innumerable modifications recorded by Leonardo is indeed a measure of his mental power and achievement. And yet it might be that some sections in his book VI are more easily understood if they are seen against the background of this tradition. The method which medieaval painting had inherited from antiquity and passed on to the early Renaissance relied basically on the three stage method which Cennini also advocates for the modelling of drapery. The crown of the tree was painted in dark green and on this ground branches and leaves were painted in layers with the lightest in front. Needless to say the formula was enriched and modified in a variety of ways which could only be fully analysed with the help of faithful colour plates, too expensive to produce in such numbers. They would show, for instance, that account was taken of the fall of light so that the gradation towards brightness was shifted from the centre towards the source of the illumination,¹⁸ how the elements were multiplied and refined, and variations were introduced to indicate various types of trees.

It is in this development that Leonardo inserts himself. In fact his systematic statement can best be understood in this context (TP McM 910, CU 251 v):

Of the states of the foliage of trees. Four states of the foliage of trees are: luster, light, transparency, and shadow. If the eye looks down on this foliage, the illuminated part will seem to be greater in quantity than the shadowed part, and this occurs because the illuminated side is larger than the shadowed side and is subject to light, high light and transparency. For the moment I shall leave transparency aside and describe the appearance of the illuminated part, which is considered the fourth of the qualities of colors which vary on the surfaces of bodies. This is a medium quality, which means that it is not the principal light but a secondary one, which is accompanied by a secondary rather than a principal shadow. This medium, illuminated part comes between the high light and the shadowy quality which, in turn, comes between the medium illuminated part/and the principal shadows.

The third state, which is transparency, occurs only in transparent objects, and not in relation to opaque bodies. But I am speaking, at present, of the leaves of trees and it is necessary to describe the second state of foliage which is important in depicting plants. This has not been employed by anyone before me insofar as I am aware. It is, as has been stated, the portion of the foliage situated in the lower part of the tree.

It seems to me that in speaking of “the medium quality, in other words not the principal light but a medium one” Leonardo was addressing painters who were used to the traditional methods of gradation in the rendering of light and shade.

Having thus established his ground, however, he reminds them of the incompleteness of the schema, the effect of transparency, an effect which, as he explicitly states had never yet been employed in the rendering of foliage.

It looks as if Leonardo here exceptionally referred to his achievement as a painter, claiming that nobody before him had ever painted the transparency of leaves.¹⁹ There are few passages in the Trattato where the identity in Leonardo's mind of what we call ‘art’ and what we call ‘science’ is more clearly brought out. He writes not only as a painter but also as a scientist, who has discovered a new variable that must be taken into account in the description and explanation of effects, though he himself is not yet quite at ease with the consequences of this discovery. In fact it will be noticed that the four states “accidenti” he enumerates at the outset of the paragraph quoted are not really on the same level. Light “lume” and shadow “ombra” are really dependent on one variable, the rays of the sun or their reflection from the sky, ‘lustre’ “lustro” depends in addition on the quality of the leaf, whether it is smooth or mat, and transparency again both on the intensity of the light and the thickness of the leaf.

Such slight inconsistencies are not rare in the section, and yet, I believe, they would present no obstacle to a detailed commentary such as I envisage. It might then turn out that here as elsewhere Leonardo had set himself the aim of an Euclidian demonstration more geometrico, classifying and tabulating one of the most elusive and varied visual phenomena in our environment on the basis of a strictly limited number of elements. Admittedly an understanding of his intention is not helped by the jumbled sequence in which his observations are presented in the Codex Urbinas. Here, as elsewhere, the editor of the standard English edition, A. Philip McMahon, has attempted to bring order into this confusion by radically re-arranging these paragraphs, but the reader who goes through the relevant notes from 910 to 980 will be more impressed than enlightened. Not that any arrangement can be expected to work, there are too many repetitions, overlaps and complexities in the individual paragraphs to permit a perfect systematisation on the basis of Leonardo's text alone, but it seems to me that an approach through the analysis of the variables he considers might help initially to clarify his ideas and his method.

Having established frist what he calls the “vera figura” of a tree including the distribution and character of its foliage, he largely operates with three elements: the light, the eye and the air. In most of the paragraphs the light is that of the sun, for this makes it easy to plot the direction of the fall of light on the tree in descriptions and diagrams, though Leonardo characteristically advises the landascape painter (TP McM 978, CU 256) to avoid painting sunlight and rather select the diffuse light emanating from the sky alone. It is the fall of light, of course, which creates the objective condition of the distribution of shadows but it is the condition of foliage which accounts fot the places where sunlight penetrates or where leaves throw shadows on other leaves (TP McM 911, CU 253 v, Fig. 9):

Of the ends of branches of leafy trees. The primary shadows which the first leaves cast upon the second leafy branches are less dark than those which these shadowed leaves cast upon the third leaves and also less dark than those cast by the third shadowed leaves upon the fourth. This brings forth the fact that the illuminated leaves which have the third and fourth leaves which are in shadow, as their backgrounds, appear in greater relief than those which have the first shadowed leaves as their background. For example, if the sun were E, and the eye N, and the first leaf illuminated by the sun were A, which has as its background the second leaf B, I say that this leaf would stand out less, since it has as its background that second leaf, than if it projected farther out and had as its background the leaf C, which is darker because more leaves lie between it and the sun. The leaf A would stand out still more if it/had as its background the fourth leaf, that is, D.

What matters to the painter, however, is not only which part of the trees are lit, but what of these conditions can be seen from a particular vantage point: in other words appearance here, as always, depends on the second variable, the position of the eye. The simplest case is discussed in TP McM 920, CU 262:

Of plants in the south. When the sun is in the east, trees in the south and north have almost as much light as shadow. The amount of light is greater the farther west they are, and the amount of shadow is greater the farther east they are.

By “the amount of shadow” Leonardo means of course visible shadow, as he also demonstrates in a diagram TP McM 968, CU 262 v (Fig. 10):

Of landscapes in painting. Trees and mountains represented in painting should show shadows from that side of the picture from which the light comes and their illuminated parts should be shown from that side from which/the shadows come, so that the light and shadow is shown in those parts where the eye sees both light and shadow. This is proved by the figure in the margin.

The a priori character if this observation is neatly brought out in TP McM 922, CU 259:

Of the shadows of trees. Shadows of trees in landscapes do not appear to be the same in trees at the right as in those at the left, particularly when the sun is at the right or at the left. This, proved by the fourth proposition which states: opaque bodies lying between the light and the eye appear to be entirely in shadow, and also by the fifth proposition which states: the eye placed between the opaque body and the light sees the opaque body all illuminated; and furthermore by the sixth proposition which state: if the eye and the opaque body are between the darkness and the light, the opaque body will be seen half in shadow and half in light.

It is from this rational schema that Leonardo proceeds to qualify and refine the explanation of appearances as in TP McM 921, CU 254 v:

Of the shadows of trees. When the sun is in the east, the trees to the west of the eye appear to have very little, almost imperceptible, relief if the air which lies between the eye and the trees is very dusty. According to the seventh proposition of this book,/these trees are without shadow, and although there is shadow in each division of the foliage, it happens that because the visual images of the shadow and the light which reach the eye are confused and intermingled, and because of the smallness of their size and form they cannot be envisaged.

The principal lights are in the middle of the trees, and the shadows toward the edges, and the spaces between are marked by the shadows in these intervals when the forests are thick, but when they are sparse the outlines are but little to be seen.

More possible permutations are introduced in such paragraphs as TP McM 916, CU 255:

Of the sizes of shadows and lights on leaves. The branches and foliage of trees are seen from below, above, or from the center. If they are seen from below, then the light will be universal and the shadowy part will be greater than the illuminated part; if they are seen from above, the illuminated part will be larger than the shadowed part, and if they are seen from the center, the illuminated part will be as large as that in shadow.

The two primary variables, let us call them the position of the sun and that of the eye, will of course also effect the two “accidents” Leonardo had enumerated in TP McM 910, CU 251 v, transparency and lustre. Clearly the first of these will be strongest if we see the leaf against the sun as Leonardo specifies in two annotated diagrams in which he now distinguishes the three “accidents” of shade, lustre and transparency, where the leaf is shown to be transparent exactly at the point between the eye and the source of light (TP McM 939, CU 261 and 261 v, Figs. 11 and 12a):

Of the shadow of the leaf. Sometimes a leaf has three incidental conditions; that is, shadow, high light, and transparency; for example, if the light were from N on the leaf S, and the eye at M, it would see the part A illuminated, B shadowed, and C transparent.

The leaf with a concave surface, seen in reverse from below, sometimes appears half shadowed/and half transparent. For example, let P O be the leaf, and the light M, and the eye N, which sees O shadowed because the light does not strike it at equal angles on the right side nor on the reverse, and let P be the right side, illuminated with a light which shines through to the reverse side.

Even here, though, these simple conditions are complicated in reality by the overlap of foliage as Leonardo demonstrates in one of his most beautiful diagrams, TP McM 927, CU 260 v (Fig. 12b):

Of dark leaves in front of transparent ones. When leaves are placed between the light and the eye, then the leaf nearest the eye will be the darkest and the most distant will be the brightest, provided that it does not have the air as its background. This happens in the case of leaves which are beyond the center of the tree, that is, toward the light.

No wonder here, as with the effects of sunlight as such, Leonardo interrupts his demonstrations to warn the painter against incorporating them in his paintings (TP McM 977, CU 260):

Of never portraying foliage transparent to the sun. Never represent leaves which are transparent to the sun. These are indistinct, because on the transparency of one leaf there will be impressed the shadow of another leaf which is above it, and this shadow is sharply bounded and of a determinate density and sometimes it takes up a half or a third of the leaf that is shadows, and thus that cluster is confused and its portrayal is to be avoided.

When it comes to the question of lustre Leonardo is more specific about the need to consider the quality of the leaf itself. “that leaf which has a hairy surface does not have sheen” he observes (TP McM 972, CU 268). But in generally considering the effect of lustre Leonardo introduces his third main variable, the “air”. We have seen what important role he assigned to this element in his discussion of mountains and once more it is the air as much as the light which accounts for the modification of appearances.

Again he relates these effects to general propositions to which he refers more geometrico (TPMcM 936, CU 256 v):

Of the high lights on the foliage of trees. The leaves of trees commonly have a polished surface, because of which they mirror in part the color of the air, and because the air takes on white since it is mixed with thin and transparent clouds, the surfaces of the leaves, when they are naturally dark, like those of the elm, and when they are not covered with dust, have high lights which appear to be blue. This happens according to the seventh proposition of the fourth book which shows that brightness compounded with darkness produces the color blue.

These leaves will have high lights that are bluer as the air which is mirrored in them is more pure and blue, but if these leaves are young, like those at the tips of branches in the month of May, then/they will be green tending toward yellow, and if their high lights are produced by the blue air which is mirrored in them, then these will be green according to the third proposition of the fourth book which states: the color yellow mixed with blue always produces the color green.

The high lights of all the leaves of a dense surface will take on the color of the air, and the darker the leaves are, the more they will function like mirrors, and as a consequence these high lights will become blue.

Leonardo, of course, is the pioneer of ‘aerial perspective’ so the effect of distance on the shape and colour of trees is attributed to the amount of air intervening between the eye and the object. Once again he aims at mathematical precision in describing the loss of detail with distance (TP McM 950, CU 253):

Branches and twigs of trees at diverse distances. The trees in the first plane show their true shapes to the eye, and each cluster of leaves growing on the last twigs of the trees shows sharply its lights, high lights, shadows and transparent parts; in the second plane within the distance from the horizon to the eye, all the clusters of leaves appear like points on the twigs; at the third distance all of the twigs seem to be points sown among the larger branches; at the fourth distance the larger branches are so diminished that they remain only as indistinct shapes in the tree; then follows the horizon, which is the fifth and last distance, where the tree is completely diminished in such a way that is like a point in shape. Thus I have divided the distance from the eye to the true horizon, which ends on a plane, into five equal parts.

In another paragaph which is rightly famous he goes further still and relates the gradual change of apparent shape to the effect of the intervening air (TP McM 956, CU 258 v, Fig. 13):

What outlines distant trees display against the air which forms their background. The outlines which the branches and foliage of trees display against the illuminated air, have a shape tending toward the spherical the farther distant they are, and the nearer they are, the less they show of this spherical shape. For example, A, the first tree, because it is near/to the eye displays the true shape of its branching, which almost disappears in B, and is entirely lost in C, where, not only are the branches of the tree not seen, but the whole tree is recognized only with great effort.

It is in this context also that he formulates what might be called the law of diminishing tonal contrasts which had been known empirically to the ancient world and which he himself had applied with such powerful effect in his landscape background (TP McM 951, CU 258/258 v):

Of the luminous part of the verdure of plants. Luminous parts of the verdure of trees in the vicinity of the eye appear to be brighter than those of more distant trees, and their shadowed parts appear darker than those of the more distant trees.

“The bright parts of distant trees are darker/than those of adjacent trees, and their shadowy parts seem brighter than the shadowy parts of those adjacent trees. This comes about because the concurrence of the visual images becomes confused and mixed on account of their great distance from the eye that sees them.

The commentary to the full text, such as I envisage it, would not, of course, have the drawback which this relatively small selection inevitably exibits. It would be able to do justice to the richness and subtleties introduced by Leonardo in his countless modifications of the principal schemata. These modifications would then be seen to range from wholly subjective to purely objective phenomena. By subjective I mean those responses of the eye which are due to the effects of contrast or dazzle, effects which always fascinated Leonardo, as in the following observation which forms part of TP McM 973, CU 259 v: “When one green is behind another green, the high lights and transparencies of the leaves appear to be of greater power than those which border on the brightness of the air”, an effect which must be due to that “brightness” appearing to tone down the highlights more prominently in evidence towards the centre.

Among the objective phenomena those would have to be considered which are due to passing physical conditions, as where Leonardo describes the effect of wind (TP McM 946, CU 266) which renders the appearance of trees brighter “because each leaf is paler on the revese side than on the right side”, a condition not necessarily universal. Other phisical modifications are strangely neglected, such as the effect of rain or dew with their sparkle, or the change of the colour of foliage in the autumn. The fact that these points are omitted and that conifers are nowhere mentioned in this section, though some species are referred to in other parts of the treatise, underlines the fragmentary character of the notes we have. Leonardo, of course, was aware of the fact that the topic was inexhaustible. He made a brief reference at least to the effects of chance, or as he puts it, the “mixture of accidents”, but being Leonardo he even wished to quantify its working (TP McM 965, CU 258 v):

Of the appearance of chance conditions. At a distance there is produced a mixture of the chance conditions affecting the foliage of trees. The foliage participates more in such chance conditions which is largest in mass.

Enough has been said, though, to make the point that Leonardo's review of visual phenomena of which the landscape painter should be aware is not ‘inductivist’ in the sense of a naive Baconian ideal of science based entirely on first-hand observation. On the contrary, true scientist as he was, he used a deductive model starting with first principles but testing his deductions in the world around him. It is in the nature of such a procedure that it remains wedded to universals, to categories and classes of phenomena, in other words to visual schemata, however articulated. The individual and unique will never be matched through such a procedure. Maybe it was an awareness of this limitation which made the compiler close his account with a note which boldly disregards the variability of appearances which had been the subject of the preceding pages (TP McM 981, CU 268):

Precept, for imitating the color of leaves. Those who do not wish to rely entirely on their own judgment in duplicating the true colors of leaves, should take a leaf from that tree which they desire to represent, and mix their color based on this, and when there is no difference between their color and the color of the leaf, than you can be certain that your color is an exact duplication of the leaf, and you can do the same with other objects that you wish to represent.

Did Leonardo pen this note tongue in cheek, or did he perhaps want to return to base and remind the painter of the elementary facts of ‘matching’ which underlies even the study of the countless modifications he has set himself to investigate and to explain? He knew that appearances cannot be fully matched in painting, because the world we see with two eyes is three dimensional while paintings are flat,²⁰ and also because the painter only has white pigment to emulate the brightness light, a fact first observed by Alberti.²¹ If we are to believe Vasari it was this handicap of the painter which drove Leonardo to adjust his scale towards the dark end of the spectrum to enhance his contrasts.²²

Today we have mechanical methods of matching nearly all effects of nature; the camera which can be enhanced by the borrowed light of projectors not to mention the stereo effects achieved by holographs. Even without these additional devices colour photography has reached a degree of perfection which no longer allows us to dismiss the varied images we find in the best books on natural scenery with a superior smile. Leonardo certainly would not have done so. He would have scrutinized these records with infinite patience to supplement his observations. An ideal edition of the Trattato might well do the same, confronting Leonardo's verbal descriptions and diagrams of trees, leaves or mountain scenery with carefully selected images which would show both the accuracy and the limitations of his research.

It certainly needs an effort of the imagination to envisage such a sequence of illustrations. For the section would present us with panorama of nature wholly remote from the landscape not only of Leonardo himself but also from that of his fellow Italians. Would not a naive reader of these descriptions of scenery and atmospheric effects rather conjure up in his mind something like the backgrounds of Netherlandish paintings? Leonardo certainly knew the Portinari Altar by Hugo van der Goes and must have been impressed by it. But the notes for the sixth book of the Trattato nearly all date from the sixth decade of his life.²³ Do they testify to yet another attempt to enrich his creativity and to create a new kind of painting? The famous sketch of a copse at Windsor (12431) is a precious testimony to his effort to link once more his theory with his practice, but miraculous as it is, it remains isolated and enigmatic. Maybe his theoretical ambition had once more overtaken his practical reach and made him still more “impatient of the brush”.²⁴

Owing to an accident of history the version of the Trattato on which the editio princeps was based, lacked most of the observations here discussed. The development of landscape painting occurred therefore rather independent of Leonardo's heroic effort. In fact more than three hundred years had to elapse before his enterprise was taken up again, in that astounding catalogue of visual phenomena, Ruskin's Modern Painters.²⁵ His beautiful chapter ‘Of Truth of Vegetation’, like the other pages of that multivolume work, is conceived as a defence of Turner and a vindication of his fidelity to Nature. The author is at pains to contrast his hero with his predecessors who failed to look at the world around them. Judging by his comment there are indeed many works by famous exponents of landascape painting in which the laws of the morphology of trees which Leonardo had established are flouted. To compare Ruskin's observations with those of Leonardo would altogether be a rewarding task but like the other desiderata here outlined it has to be left to another generation of scholars.

Notes

1. Leonardo da Vinci, Treatise on Painting (Codex Urbinas Latinus 1270), Translated and annotated by A. Philip McMahon, with an Introduction by Ludwig H. Heydenreich, Vol. I. Translation, Vol. II, Facsimile, Princeton University Press, Princeton (New Jersey), 1956.

2. Carlo Pedretti, Leonardo da Vinci ‘On Painting’, A Lost Book, (Libro A), University of California Press, Berkeley and Los Angeles, 1964; and, the same, The Literary Works of Leonardo Da Vinci, A Commentary to J. P. Richter's Edition, Phaidon Press, Oxford, 1977.

3. J. J. Gibson, The Ecological Approach to Visual Perception, Houghton Mifflin Company, Boston, 1979.

4. Art and Illusion, A Study in the Psychology of Pictorical Representation, New York and London, 1960. Italian Translation Arte e Illusione, Torino, 1965.

5. In my Gerda Henkel Vorlesung of 1982, Ideal und Typus in der Renaissancemalerei which is due to be published soon in Duesseldorf. I have attempted to place this phenomenon into a wider context.

6. See my essay Leonardo's Grotesque Heads originally published in Achille Marazza, (ed.) Leonardo, Saggi e Ricerche, Istituto Poligrafico dello Stato, Roma, 1964, and reprinted in my The Heritage of Apelles, Studies in the Art of the Renaissance, Phaidon Press, Oxford, 1976.

7. Ludwig Golscheider, Leonardo da Vinci, Landscapes and Plants, Phaidon Press, London, 1952. Giorgio Castelfranco, Il paesaggio di Leonardo, Studi Vinciani, Roma, 1966.

8. See my The Heritage of Apelles, cit., p. 33.

9. See my The Heritage of Apelles, cit., pp. 11-13.

10. Giorgio Castelfranco, Sul pensiero geologico e il paesaggio di Leonardo, Studi Vinciani, cit.

11. See the Madrid Codex II (ed. L. Reti).

12. Richter, The Literary Works, as quoted above, 300, Lei 4 r (Ham 4 A).

13. Cennino D'Andrea Cennini, Il libro dell'Arte, edited by Daniel V. Thompson Jr., Yale University Press, New Haven, Cap. LXXXV.

14. See for instance the apse mosaic of S. Apollinare Nuovo in Ravenna, as discussed in my The Heritage of Apelles, cit., on p. 12.

15. According to André Chastel in The Unknown Leonardo edited by L. Reti, London, 1974, p. 147, Leonardo applied this rule in the magnificent mountain panorama of W 12410. But did the artist here not rather wish to isolate the soaring peaks which were the main object of his interest?

16. See my Art and Illusion, cit., Ch. V, section Iv.

17. As cited in ref. 13, Cap. LXXXVI.

18. An early example is Taddeo Gaddi's fresco of the Annunciation to the Shepherds in Santa Croce, Firenze.

19. The foliage seen against the sky on top of the Louvre Madonna of the Rocks may be an early example; but Leonardo may have been anticipated by Piero della Francesca in whose Baptism of Christ (London, National Gallery) the radiance is seen behind the leaves of the tree.

20. TPMcM 220, CU 46 v and 47, as discussed in my Art and Illusion, cit., Ch. III, section 1.

21. L. B. Alberti, De Pictura, ed. C. Grayson, Phaidon Press, London, 1972, II, paragraph 47.

22. See for this interpretation my The Image and the Eye, Oxford, 1982, p. 229.

23. Nearly all the notes used in section VI come from Ms G and are dated by Pedretti in his Commentary on Richter (cit.) around 1510.

24. The quotation is from Nuvolara's famous letter to Isabella d'Este of April 8th 1501.

25. First published in 1843; I know of no evidence that Ruskin at that time knew the full text of Leonardo's Trattato.