Introduction to the Post on “local colour interactions”
This Post is the second that offers a link to a .PDF version of a chapter from “What Scientists can Learn from Artists”. The purpose of making this more scientifically oriented information available on this website is to encourage readers to go deeper the ideas presented in my practice-oriented books on drawing, painting and creativity (see “Contents List”on the “Posts Page”). Chapter 11, the chapter featured here, focuses on new and unfamiliar things of potential value to artists that can be said on “local colour interactions”, a subject that has featured widely both in books and in the classroom. Its father figure is Eugene Chevreul, the Chemist at the Goblin tapestry works, who was responsible for the phrase “simultaneous colour contrast”, and the best known publications on the subject are by Johannes Itten and Joseph Albers, both teachers at the Bauhaus. Of more recent books covering the subject, I can recommend “Colour : A workshop for artists and designers”, by David Hornung.
With so many authoritative writings on the subject, it might be supposed that I would have little to add, particularly since, as a general policy throughout my books, I have done my best to avoid wasting time on subjects that have previously been exhaustively covered in convincing ways. It is for this reason that my chapter on “local colour interactions” concentrates on subjects that do not appear in the publications of Chevreul, Itten, Albers, Hornung or, as far as I know, of anyone else. What I have to say is based on research triggered by the excellent teaching I received at my art school and issues arising in my own paintings. Its novelty comes either from original or less well know scientific research that deals with matters of potential interest to artists.
It was unequivocal evidence of “induced colour” and “colour constancy” that triggered the realisation among scientists towards the end of the Eighteenth Century that colour is not a property of surfaces in the external world but phenomenon that is made in the head. Once this idea had been digested, it gained momentum and evidence began to pour in to suggest that all visual experience is a creation of the eye/brain combination. This game-changing paradigm shift was to lead, not only to the birth of the science of “visual perception”, but also to fundamental changes in the practice of artists, either when drawing or painting from observation or when seeking control of pictorial dynamics. This is why the “constancies” and “simultaneous contrast dynamics” play such an important role in my books on the practice of painting and drawing. It is also an important part of the reason why I have written “What Scientists can Learn from Artists”, the last volume of my four volume series that explains the science behind so many of the ideas elaborated upon in the remaining three volumes. In going more deeply into the subjects that play such an important role in these books about artistic practices, it plunges us deep into the astonishing nature of the working principles of visual perception. Apart from the sheer wonder this must surely generate, knowing about the ways these determine how we “look” and how we “see” should have a significant benefits for artists: The deeper understanding and appreciation of the extraordinary things that are happening in our heads should help artists to:
Deal with the many practical problems that invariably face them when drawing or painting from observation
Make more creative use of their physical and conceptual tools.
The next Posts I will be chapters from the science volume.
A life changing event
This Post on “colour constancy” is the first from “What Scientists can Learn from Artists”. Its inspiration derives fromEdwin Land’s irrefutable demonstration of the phenomenon of “colour constancy”, which proved to be a milestone in the search for an understanding of a subject that turned out to be of key importance to the understanding of how we “perceive surface”, “sense space” and are “aware of of lighting conditions”, all subjects of key importance to the ideas presented in “Painting with Light and Colour”.
A photo of the equipment used by Land for his epoch making colour constancy demonstration.
A reprise of the “Introduction” to the chapter and a link to a .PDF version of it (no need to read it twice: if you read it below, you can skip it in the chapter)
Links to Posts from “Painting with Light and Colour”, all of which (particularly chapters 7 to 11) have a debt to research that grew out of the colour constancy demonstration.
As explained earlier, a key event in my life was the encounter with Professor Marian Bohusz-Szyszko. The ideas he shared set me off on a lifelong journey of discovery. My first step was to set about testing his seemingly extravagant assertion that it is only necessary to follow two rules to guarantee a good painting:
There must be no repetition of colour on the same picture surface.
All the colours used must be mixtures containing at least a trace of complementary.
After four years of experimenting, I proved, at least to my own satisfaction, that there is a special quality in all paintings that abide by these two rules. It is difficult to describe, but it involves the creation of a sense of pictorial space and harmony.
Fortunately, a troubling paradox arose that would eventually have a profound effect on the development of the ideas presented in this book. It concerned the Professor’s physics-based proof of the invariable variability of colours in nature. This asserted that no two parts of any surface will reflect exactly the same wavelength combinations into our eyes due to:
The complexity produced by the inter-reflecting surfaces
Variations in viewing angles and distances
The paradox is that, if the light reflecting from two parts of a surface can never be characterised by the same wavlength combination, how could artists repeat colours on a picture surface? Even if two regions were painted with exactly the same pigment-colour, how could these appear as the same?
Other people might already have known how to resolve this mystery, but for many years I had no idea how to do so. My first inkling of a solution came after many years, as a result of reading a paper by Edwin Land, the inventor of the Polaroid camera. In it was a powerful demonstration of the phenomenon of “colour constancy” and an attempt to explaining it. What the demonstration showed was a region of colour within a multicoloured display (henceforth referred to as the MCD) being perceived as remaining the same, even when the experimenter changed the combination of wavelengths being reflected from it. I was excited because here were two colours being perceived as the same despite reflecting different wavelength combinations into they eyes? For me it was a eureka moment. However a big problem emerged for it was soon clear to me that the explanation of the colour constancy demonstration suggested by Land was not neurophysiologically plausible. An alternative had to be found. I could never have guessed at the treasure trove of discoveries that would come out of my struggles to provide it. This chapter describes Land’s demonstrations in the context of an earlier attempt at explaining colour constancy. The next chapter introduces our neurophysiologically plausible colour constancy algorithm.
This Post provides a link to “Colour mixing by layering”, Chapter 15 of my book “Painting with Light and Colour”. It is the last and longest of the five chapters devoted to colour mixing. Below is a slightly edited reprise of the “Introductory” to the chapter. If its contents make you want to find out more, click on the link beneath to obtain a .PDF version of the whole chapter.
This long and important chapter deals with the practical problems and opportunities that arise when coats or washes of one pigment colour are layered or otherwise superimposed on coats or washes of other pigment colours. In particular it is about how outcomes are influenced by the degree of translucency or opacity of the pigment-colours used. An important conclusion is that the same factors that are at play in colour mixing by layering are also at play in colour mixing by stirring.
The chapter starts by using the example of oil paints to illustrate general principles that apply to all colour mixing. It progresses to an analysis of ways in which colour superimpositions play out in the cases of watercolour, gouache, acrylic and dry pastel. The chapter concludes with some supplementary remarks on the use of scumbling and glazing in oil paints and acrylics.
This Post provides a link to Chapter 14 from my book“Painting with Light and Colour”, which is the fourth of the five chapters devoted to colour mixing. Its purpose is to show that all the complications of colour theory proposed and explained in the previous chapters, need not be a barrier to our creativity when it comes to their practical application. Quite the reverse. Below is a reprise of the “Introductory” to Chapter 14. If its claims make you want to find out more, click on the link beneath it to obtain a .PDF version of the chapter, which will explain how it can be made easy (a) to mix and (b) to make use of any of the thousands of subtly different, complex colours required for exploring the full extent of colour space.
This Chapter consists of two parts. In the first, a suggestion is made as to why people have been daunted by what they initially perceive as practical difficulties inherent in the approach to colour-mixing adopted in this book. In the second, a description is given of a practical way of getting around the seeming obstacles. At first sight this may appear to involve important sacrifices, but upon further investigation it turns out that even its shortcomings can be interpreted as powerful advantages..
This Post provides a link with Chapter 13, the third of the five colour mixing chapters I promised to publish in the coming days. Its title is “Finding a maximum of colours”. As with the others Posts on colour mixing, I start by providing an edited version of its “Introductory”.
Estimates have been given as to the maximum of colours that can be made from mixtures of the paints available to artists. The smallest of these suggests hundreds of thousands. At first sight such huge numbers might seem to be beyond the reach of artists. However, Chapter 13 explains the theory of why this is not the case, while Chapter 14 shows how easy it is to use the theory in practice. You can read Chapter 13 by clicking on the link below. Chapter 14 will be made available very shortly.
This is the second of five posts on colour-mixing. Its purpose is to prepare for the three more practical chapters that follow, by putting straight a number of commonly held misunderstandings relating to the ‘colour circle’. Below is a slightly edited version of the Introductory to the chapter, followed by a link to it the whole text.
At my painting school, I give talks on colour-mixing. The chapter which I am publishing in this Post as well as the next three chapters (all of which I will publish in the near future) are based on these. Between them they both flesh out some of the claims made in the previous chapter and provide a sound and practical approach to colour-mixing. My purpose is to provide help with:
Finding a maximum of colours in any part of the colour sphere (as described in the last chapter).
Creating a sense of light, space and harmony in paintings.
The first of my talks concerns colour-mixing by stirring (as opposed to colour mixing by layering, which I will deal with later). Experience has shown that for many people coming to my Painting School for the first time, it is necessary to start my explanations at the most basic levels. Accordingly, I introduce my talk by apologising in advance for going over ground that may already be familiar to some, but suggest that it is better to be absolutely sure of building on common and solid foundations.
Recently I was asked if I could post the five colour mixing chapters from my book “Painting with Light and Colour” (Chapters 11 – 15). I will be surprised if you do not find that many of the ideas in them are new, interesting and practical. At the bottom of the page is a link to Chapter Eleven, the first of the four chapters, whose title is, “Colour mixing – definitions and misconceptions”. To whet your appetite (below the image) I have included a slightly edited version of its “Introductory”.
Introductory to Chapter Eleven
Introductory At the outset of my life as an artist, my conception of colour-mixing was of a dry and mechanical subject. I thought of it as no more than one of those necessary basic skills that could easily be picked up along the road. To my surprise, nothing turned out to be quite so routine as it had seemed, and one line of enquiry led to another in a most seductive way. Each new development plunged me deeper into the history either of science or of art, until an engagingly coherent story emerged. The result was a practical understanding of a kind that might be difficult to find elsewhere.
“Most how-to-do-it art books have sections on colour-mixing and there are a number of tomes that offer technical information for professionals. These latter tell us that scientists have understood the physics underpinning colour-mixing theory for a very long time: Certainly they have done so since James Clerk Maxwell’s lecture on colour vision, given at the Royal Institute, two years before the First Impressionist Exhibition in 1874.
In view of the availability of all these sources of information, it might be thought that there is nothing left to add. Unfortunately, this is far from the case. The problem is that:
Too many painters are being seriously misled by the half-truths and even falsehoods which have entered into the stock in trade of popular colour-mixing theory. Science has far from stood still since the 1870s.
Particularly since the 1970s, scientists have been finding out a great deal of new information about how eyes and brains work and, as a result, have arrived at a number of new understandings that could help artists in practical ways, which are not being made use of by the artistic community.
For these reasons and others, it is clear to me that there is a need for the up-to-date approach to practical colour mixing that is supplied by the next chapters.
One approach to clarifying matters is to place the information presented in an historical context. Doing so reveals that:
Some of the best of ideas have been obscured by the passage of time.
The evolution of colour-mixing theory, owes much to parallel development of the histories of science and of art.
The story of when, how and why artists adopted new colour-mixing practices, provides many insights into their potential uses in painting.
With respect to the links between the discoveries of the scientists of visual perception and the practice of the artists, the evidence is usually sparse and often ambiguous. To compound the problem history (not least the history of science) becomes distorted because it is told by people who write with the benefit of hindsight and sometimes from the perspective of a particular prejudice.
It may surprise some people to find how many famous scientists are credited both with more originality and much more fully developed and rounded versions of their ideas than they actually had. A mismatch of this kind may be suspected in the relation between the confusions inherent in the early development of the ideas developed by Seurat and Cézanne and the neat synthesis of them by Professor Bohusz-Szyszko. Similarly it is unlikely that any of the early Impressionists had as clear a conceptual framework concerning the real surface/illusory space dynamic as was eventually to evolve from their pioneering ideas. While these are very interesting areas for discussion, the process of trying to unearth and pin down exactly what the early pioneers had in mind is a work for scholars. The focus of this book is artistic practice and it is the more refined picture as developed by the more recent artists and theorists that are the most useful in terms of their practical value.
We start a short survey of these by providing some basic definitions as used in this book:
The 2019 season is over. Despite having to cope with exceptionally hot weather, both sessions went well. A number of old stagers declared their session to be “the best ever“. New students, who came here from either USA or Scandinavia, were equally enthusiastic, most vowing that they would be returning in 2020. It is always a pleasure to see returnees many of whom have become valued friends and the new dynamic brought by newcomers is always welcome. One innovation was giving my talks towards the end of the day and restricting them to a maximum of only half an hour.
Two innovations were forced on us:
As Hélène was not available to provide breakfast at her house, we started the day at La Baladin de la Gresigne, with its capacious terrace and its panoramic views (see photos below). Food was served by the much appreciated Frederique.
Angela, who has cooked marvelous mid session meals for nearly thirty years, is not so young as she was and decided to call it a day. As a replacement we decided to take students to a restaurant. We chose La Cabanon in the neighbouring medieval hilltop town of Puycelsi. While all who knew Angela will have missed her cooking, the innovative menu produced by the young chef at La Cabanon was much enjoyed and, as a bonus, we benefited from the chance to do a little tourism. Nobody will be surprised that we plan to do the same combination next year.
As usual my daughter Katherine provided the meals on the first, second and last days. As usual, her her beautifully presented and delicious offerings received many compliments.
For your enjoyment I have gathered a small selection of photos that students and others provided over the summer and I have made a slideshow of them. Sorry there are important omissions but that is what happens if you rely on others with their own agenda. Apologies to anyone who has been left out. It is hard to focus on taking photos and teaching at the same time.
Next year we have to face up to the fact of Sarah’s departure on a two year MFA course in Boston University. She has been a wonderful help for me and to students over many years now. To replace her as best I can, I have invited two former students to help with the teaching. Both studied with me early on in their career as artists. Cathy Layzell first experienced my teaching some twenty years ago when I was teaching sessions in Norfolk, England. Later she came out to Montmiral a number of times. She is now an established artist living in South Africa. Ken Marunowski came to Montmiral for the first of several sessions some ten years ago, before deciding to dedicate his life to painting and is now an exhibiting artist. Both have experience of teaching. A paragraph and photos introducing them now replaces the one about Sarah at bottom of the “Structure of Workshops” page of this website.
Finally, we have updated the “Booking Information” page. To meet additional costs, we have decided to to increase the price of coming by €30. We have also decided on the dates for next year, which are slightly different to previous years:
Saturday 20 June – Saturday 4 July 2019
Saturday 18 July – Saturday 1 August 2019
Notice there will only be two weeks, instead of three, between sessions. One student pointed out that it will make it easier for those like her who wish to come for two sessions.
I look forward to seeing as many of you as possible next summer.
Some more photos for your enjoyment
The other day I took some photos from the terrace of the “Baladin de la Gresigne” where the students now meet for their breakfast. The sunflowers are no longer in their full glory but I think you will agree that the views remain splendid. The photo of the facade was taken some while ago.
When I arrived at art school in the late 1960s, ambiguity was one of the first things we were expected to think about. Over the years I have learnt what an important role it has played in artist’s thinking over the last on hundred and fifty years.
Ambiguity occurs whenever two or more interpretations are in competition with one another. They may be within any domain of sensory experience or across domains, but here it is considered in relation to the visual domain. The purpose of this Post is to present some of the reasons why ambiguity should be of fundamental interest to artists, particularly if they are:
Wish to depict illusory pictorial space.
Seeking to create either harmony or discord in their paintings.
An illustration of ambiguity
A good example of a high level of ambiguity is provided by the well known vase/face illusion illustrated in Figure 1, in which we can see either a vase or two silhouetted faces. However, it is important to notice that, although we can choose between the vase or the two faces interpretations, we cannot stop the ambiguity of the situation providing a degree of tension. The force of this can be sensed by comparing the right hand side face with the identical face in Figure 2 in which there is no left hand side face to create the vase shape. Clearly, by removing the ambiguity between the vase and the two faces interpretations, the face is easier to look at. The question for artists is whether they want to maximise ease of looking or to create works with some degree of tension.
As explained below, it can be argued that all paintings exhibit an ambiguity between their pictorial contents and their presence as objects with real surfaces. The the only way the escape this is to make use of one or more of the strategies that are available to us for that purpose. Not counting that of looking away altogether, these depend on concentrating attention on details at the expense of wider context, which can be done either by focusing down or by moving closer. However, although both these manoeuvres work well enough for reducing ambiguity in everyday visual perception, our eye/brains can seldom completely exclude the influence of alternative interpretations in paintings..
Real picture surface versus illusory pictorial space
One type of ambiguity has had a pivotal place in the history of painting. It is that between perceptions of an illusory pictorial space and awareness of the real picture surface. Historically speaking, it became important when the Impressionists and other early Modernist Painters were looking for ways in which their hand-painted images could combat the threat posed by the high levels of realism produced by the newly available photographic method. They feared that the fact that it was so easily and quickly obtained in photographs, would undermine their livelihood.
Luckily for them and for the history of painting, they saw photographic images as deceiving the eye and hit on the now seemingly absurd idea that this deception was morally reprehensible (an idea that was still influencing artists in the 1960s and beyond). Abruptly, for progressive painters at least, the trompe-l’oeil, which for so long had been the goal of artists, became something to be avoided at all costs.
The solution found by these Modernist painters was to introduce ambiguity. Their idea was to provide a counterpoint to perceptions of illusory pictorial space by emphasizing the reality of the picture surface. The painting, by Berth Morisot, illustrated in Figure 3, provides a good example of how brush marks and surface texture can be used as a means of preventing the trap of deception. What these pioneer artists reasoned was that the impossibility of being able to enter the illusory pictorial space created by the realism of the image without being aware of these indicators of real paint and real surface, meant that a salutary ambiguity would be inevitable. What they could not have guessed is that the wealth of unexplored possibilities that the picture surface/illusory pictorial space dichotomy would make available, not only to them but also to their successors during well over a century.
An example of competing cues
The Modernist Painters were also interested in another kind of ambiguity in which attention is drawn in competing directions. For example, Figure 4 shows Pierre Bonnard going a long way towards obscuring the features in his wife’s face, presumably with a view to allowing the telling gesture of the hand to take on more significance than it would have done had the eyes, nose and mouth been more clearly delineated. However, since this strategy completely fails to override the eye/brain’s built in tendency to give faces more importance than hands, the result is a pull in both directions. The consequent dynamic equilibrium is of central importance to the experience of looking at the drawing. Just as in Figure 1, it is difficult to look at the vase interpretation without being influenced by the two-face interpretation, it is difficult to look at either the hand or the face to the complete exclusion of the other.
Figurative versus abstract
Another area of competition is between pictorial dynamics based on figurative content and ones that involve non-figurative relationships such as those between colours, contours, textures, etc.. The more the Modernist Painters found their interest turning to the latter, the more they sought ways of playing down the influence of the former. A first step in this direction was that of reducing the strength of the cues that make recognition too easy (as in the case of the face in both the drawing of Marthe by Pierre Bonnard, illustrated in Figure 4, and the portrait of Ambrose Vollard by Pablo Picasso, illustrated in Figure 5). A second step was to get rid of figuration and its attention-distracting pull altogether.
Seeking to eradicate ambiguity
Many artists saw great potential in the removal of figuration but did not want to get rid of illusory pictorial space, which they felt gave extra dynamic possibilities to interactions of colour, contour, texture, in front/behind relations, etc. Others were still troubled by the immorality of deceiving the eye and sought to eliminate illusion altogether. They wanted all regions of their paintings to be perceived as being flat on the picture surface, as no doubt was the intention of Michael Kidner when making the work illustrated in Figure 6.
However, these purists were soon to realise that achieving this objective might prove more difficult than they had imagined. It seemed that no matter how hard they struggled, if a picture surface, had more than one region of colour on it, their eye/brains would find ways of creating perceptions of illusory pictorial space within it. In other words, they seemed unable to prevent some of the regions of colour appearing to be either in front of or behind others. Even the simplest two colour painting, such as the one by Ellsworth Kelly illustrated in Figure 7, would not do, for it gives at least some people the impression of a landmass, a horizon and a sky.
In the end, the only solution seemed to be one colour paintings, of which many appeared in the 1950s and 1960s. These included the series of blue paintings, embarked upon by Yves Kline in 1957, and several works by Ellsworth Kelly on the lines of Figure 8. The only ambiguity remaining lay in the question whether these were correctly classified as “paintings”. Some might think it more appropriate to describe them as “sculptures” or, merely, as “objects hanging on a wall”. For those artists who identified (in my view falsely) the search to remove the ambiguity as being of the essence of “Modernism in Painting“, it was time for “Post Modernism“.
Harmony and discord
To find out what this has to do with “harmony and discord” please consult previously posted chapters from “Painting with Light and Colour” and excerpts from the “Glossary”. In particular I suggest reading:
Today, I have been editing the entry for aerial perspective in the Glossary for my books. As I was making my corrections, I had the idea that readers of the Posts Page of this website might like a preview of this and future Glossary edits that I feel might interest them. So, to start the ball rolling, here is a slightly expanded version of the one on aerial perspective, with four images added.
Aerial perspective: Between any viewer and the surfaces of the objects at which they are looking lies a portion of the earth’s atmosphere. In addition to the transparent gases that make up air, this contains quantities of dust and other particulate matter (such as the water droplets in mist and, more evidently nowadays, various kinds of pollution). The effect of the intervening atmosphere on the appearance of distant hills and objects seen is well known to us all. We all perceive distant parts of landscapes being bluer and/or greyer and lighter than nearer parts, and objects seen through mist or fog appear progressively greyer and lighter as the distance between them and us increases. Many artists dating back to the Italian Renaissance, most famously Leonardo da Vinci (1452 – 1519) and Claude Lorrain (1600 – 1682), have demonstrated the value of applying these principles in paintings. So convincing was their effect, that they were adopted as “rules” by the French Académie Royale de Peinture et de Sculpture soon after it was founded in the mid seventeenth century. No one would dispute that the images in Figure 1 and Figure 2 produce a sense of progressive distance.
Although the theory explaining aerial perspective is scientifically sound and although it virtually always has an important effect on how we perceive both distant parts of landscapes and objects on misty days, it has no discernible effect on how we perceive objects within an arms length. So at what distance does its influence become apparent? The answer is that it varies according to the composition and density of the particles floating about within it. Thus, on dry, bright days of the kind that often follow abundant heavy rain, when the air has been washed clean, the visibility of atmospheric intervention is minimised, whereas on a hot sultry or misty days, when the air is at its fullest of dust and pollution, it is maximised. In practice, this can mean that on a clear day its effect on the appearance of objects hundreds of meters away may not be discernible.
Despite these facts of appearances, over the years, I have found that many students, when they first come to my Painting School, have been in the habit of adding blue to objects much nearer than that (in one exceptional case, a newcomer, when painting a bunch of flowers in a vase, added blue to the colour of flowers and foliage at the back of the arrangement, arguing that it made it look further away). When I see this being done, it tells me is that the student in question cannot have been looking at the near/far colour/lightness relativities. If so, how can they appreciate the amazing riches of colour relations in nature? They need to learn that rules are not for following blindly, but for testing, a process which will always open doors of awareness.
To further complicate the situation, there are a number of other variables that can result in people perceiving more distant surfaces of a particular colour as brighter and more fully saturated than nearer surfaces of the same colour: In other words, the opposite of the aerial perspective rule. For example, in summertime, the green canopies of distant oak trees that are illuminated by bright sunlight will look lighter and brighter than those of nearer oak trees should they happen to be situated in the shadow of clouds. Also, a boat on a lake that is painted with a fully saturated red that is actually further away from the viewer than a boat painted with a desaturated red will still look further away. If we made a painting of them, matching as best as possible the colours as we see them, the laws of aerial perspective would predict that the further boats would be perceived as being nearer than the nearer boat. Clearly there needs to be a way of depicting distance that has nothing to do with the representation of atmospheric intervention. Luckily there are several of these, including overlap, relative size and texture cues, but only one of them necessitates the use of colour. Unfortunately, this colour-dependent way of enhancing illusory pictorial space appears to be little known, despite its solid foundations in well known history, its sound scientific underpinning and the ease of its practical application. Much of my book “Painting with Light and Colour” is devoted to giving it new life. If you want to know more, read chapters already published on this website and watch for later Posts.
Two examples of minimal effect of aerial perspective, containing contradictions to the laws as exemplified by Clause Lorrain.
John Constable (1776 – 1837) was a great admirer of Claude Lorrain, but he looked more carefully at nature. Figure 3 and Figure 4 are images of two of his paintings that contain elements that are not consistent with a rigorous interpretation of the laws of aerial perspective. See how many you can find?
Now look at paintings by the Impressionists – Monet, Renoir Pissaro, Cezanne, Gauguin, Bonnard, etc. – to see how much they make use of the rules of aerial perspective. Where they do make use of them, was this the result of applying the rules or of looking carefully at nature? According to what is written above, far distant hills should always actually look bluer or greyer, but what about landscapes representing the kind of distances depicted by Constable or shorter ones?
Effect of patchy cloud cover on relative brightnessess
Finally to ram the point home, here are three photographs that illustrate how patchy cloud cover can produce contradictions to the laws of aerial perspective.
Figure 5 illustrates an exception to the law. Due to their being brightly illuminated by sunlight, the walls of the distant church tower are much brighter than those of the house in the foreground, which is in the shadow of passing clouds.
In Figure 6 the situation is reversed. The walls of the house in the foreground are now brightly illuminated by direct sunlight and are much brighter than those of the church tower, which is now in the shadow of passing clouds.
Figure 7 shows:
The far house,
The strip of green field in front of it,
The sunlit patches of brown earth in the ploughed field,
as being brighter than,
The near house,
The ribbon of green field in the bottom left of the image,
The area of brown earth immediately above it.
In all three images atmospheric intervention is playing a part, but in Figure 5 and Figure 7, its effects are being obscured in the ways described.