Colour Vision

A Study in Cognitive Science and Philosophy of Science

Evan Thompson

• Publisher: Routledge
• Year: 2003
• ISBN:
  9781134900794
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Cogmitive Science is a major up-coming area - very popular courses. Up-to-date review of latest research. Author was co-author of important book (translated into 4 languages since 1991). "Ecological" approach so neither "objectivist" nor "subjectivist".

1
THE RECEIVED VIEW

Newton’s laws and their later scientific corrections were used to impose on the disputed and heterogeneous phenomenon of the scattering of light, observed since the Greek physicists, the authority of the new sciences, which showed the possibility of reducing to analysis and measurement a multiple variable like the emission of light, and this with the most simple instrument in the world: a glass prism. Consequently, the individuality and the materiality of colour no longer belong to painting, or to the literature about colouring and chiaroscuro… Colours are no longer a ‘figure’ of pictorial production, but a transmission of light.

(Manlio Brusatin 1986:68–9. My translation)

Since the time of Newton and Locke, the received view among philosophers has been that colours are not found among the fundamental properties of things. Things as they are in themselves do not have colours; they have colours only by virtue of how they appear to us. Thus being coloured consists simply in being the kind of thing that looks or would look coloured to us in normal perceptual circumstances. In other words, things do not look coloured because they really are coloured; they are coloured only because they look to be so.

This view can be stated with a good deal more philosophical precision. According to Locke (1690/1975) and those who have developed his analysis (for example, Jackson 1968), colour consists in a power or disposition of something to produce sensory experiences of colour in a perceiver. The fundamental or so-called primary qualities of things, on the other hand, do not consist merely in dispositions to produce sensory experiences. According to this view, colour corresponds to a type of property that is both dispositional and subjective: being coloured consists solely in having the disposition to look coloured. Colour is therefore a relational property in two distinct ways. First and most generally, since dispositional properties typically involve relations between at least two things (for example, salt has the dispositional property of being soluble in water), colour is a relational property simply because it is dispositional. Second and more specifically, colour is a relational property because it can be specified only in relation to the visual experiences of a perceiver. Since the dispositional property in which colour consists is that of having the power to look coloured, and since something can look a certain way only in relation to the visual experiences of a perceiver, it follows that colour must be specified in relation to the colour experiences of a perceiver.

There are philosophers who hold that this conception of colour—indeed, this conception of so-called secondary qualities in general—represents an a priori philosophical discovery, rather than an empirical scientific one (McGinn 1983; Nagel 1986:75). I disagree. The conception of colour as dispositional and subjective is intimately tied to early modern science and natural philosophy, especially to Newton’s theory of light and colour. Of course, one can advocate the dispositional and subjectivist account of colour on purely philosophical grounds. Indeed, most philosophers who defend versions of the received view (for example, McGinn 1983; Peacocke 1983, 1984; Nagel 1986) do not bother to concern themselves with the scientific study of colour and colour vision. Nevertheless, the received view and Newton’s theory of light and colour grew up together. As a matter of historical fact this point would not be denied by those who defend the received view on conceptual grounds. Yet these philosophers appear to believe that conceptual considerations alone are sufficient to establish that colour must be dispositional and subjective; empirical considerations can inform us of only the contingent details. I do not accept this attempted segregation of the conceptual and the empirical, the philosophical and the scientific.¹ On the contrary, in the case at hand, I believe that the received view is deeply linked both conceptually and empirically to the Newtonian conception of colour, and furthermore that it is the Newtonian conception that actually gives rise to the modern form of the philosophical debate between subjectivism and objectivism about colour.

My intention in this chapter is to explicate and defend these remarks by examining Newton’s theory of colour in relation to the empirical and conceptual features of the received view. There are several reasons for this undertaking. First, by revealing the connection between Newton’s treatment of colour and the received view we provide the historical background and some of the conceptual basis for the discussion in the chapters to follow. Second, those contemporary philosophers who do concern themselves with the scientific study of colour vision all abandon, in one way or another, the received view: David R. Hilbert (1987) employs computational colour vision to defend objectivism; C.L.Hardin (1988) employs neurophysiology and psychophysics to defend a nondispositionalist version of subjectivism; and Jonathan Westphal (1987) employs colorimetry (the science of colour specification and measurement) and a Goethean conception of phenomenal colour science to establish real definitions of colour that are simultaneously physical and phenomenal. Finally, in the contemporary debate over the status of colour, the scientific and the philosophical, the empirical and the conceptual are inextricably linked. On the one hand, science is relevant to philosophy because the issue of the ontological status of colour cannot be separated from the issue of levels of explanation for vision (neurophysiological, psychophysical, and computational). On the other hand, philosophy is relevant to science because there are considerable conceptual issues involved in how these various levels of explanation might be related and in how they are to be applied to a specific case like colour vision. Since purely a priori treatments are unsatisfactory, and since the received view has been challenged on empirical as well as conceptual grounds, we must examine both the empirical and the conceptual features of the received view.

THE NEWTONIAN HERITAGE

Newton’s experimentum crucis

Modern colour theory owes its origin to a series of experiments performed by Newton in 1666. Newton reported these experiments in his first scientific paper, a letter written to the Royal Society of London and printed in its Philosophical Transactions, 19 February 1671/2, under the title ‘The new theory about light and colors’ (Newton 1671/1953). He also described the experiments in his Opticks (Newton 1730/1952), the first edition of which appeared in 1704.

The purpose of Newton’s letter was to establish the differential ‘refrangibility’ of light according to colour. Light, Newton argued in the first part of his paper, is not simple, but consists of rays that differ in their ‘refrangibility’—in the degree to which they are bent, or as we would say today, ‘refracted’, when they pass from one medium into another. In the second part of his paper, Newton set forth his ‘doctrine’ of the ‘origin of colors’ in thirteen numbered paragraphs. The first sentence of the first paragraph conveys the essence of Newton’s theory: ‘As the rays of light differ in degrees of refrangibility, so they also differ in their disposition to exhibit this or that particular color’ (Newton 1671/1953:74). This relation between colour and degree of refrangibility is, Newton went on to say, ‘very precise and strict, the rays always either agreeing in both or proportionally disagreeing in both’.

To appreciate Newton’s idea we need to have some familiarity with the experiments he performed. Newton first used a prism to observe ‘the celebrated phenomena of colors’, i.e., the spectrum that can be generated by a prism and a light source, such as sunlight. He darkened his room, but made a small hole in the window shutters to let in a beam of sunlight. He then placed his prism in front of the aperture so that the light would be dispersed by the prism on to the opposite wall. Newton marked seven divisions within this spectrum—violet, indigo, blue, green, yellow, orange, and red. This sevenfold division actually appears to have been based on an analogy with the seven tones of the musical scale (Wasserman 1978: 19). Most observers fail to distinguish indigo, and so would mark only six divisions.

Newton states in his letter that his research on colour was provoked by a surprise when he first used the prism. The received laws of optics at the time led him to expect that the image of the aperture on the wall would be circular, yet the image was oblong.² He then devised several experiments to explore this peculiar disproportion and its possible causes. After ruling out various possibilities, such as the thickness of the prism, irregularities in the glass, differences in the incidence of the rays coming from the sun, and curved movement of the rays after they left the prism, Newton was led to perform what he called his experimentum crucis or ‘crucial experiment’ (see Figure 1).

Figure 1 Newton’s experimentum crucis

Rotating the first prism (left) while keeping the second prism and the two boards stationary makes the complete spectrum cast on the second board move up and down, so that different portions of the spectrum fall on its aperture and pass through, where they are refracted again by the second prism. Violet is refracted the most (to V), red the least (to R), and the other colours intermediately.

Source: Sepper (1988:11)

I took two boards and placed one of them close behind the prism at the window, so that the light might pass through a small hole, made in it for the purpose, and fall on the other board, which I placed at about 12 feet distance, having first made a small hole in it also for some of that incident light to pass through. Then I placed another prism behind this second board, so that the light, trajected through both the boards, might pass through that also and be again refracted before it arrived at the wall. This done, I took the first prism in my hand and turned it to and fro slowly about its axis, so much as to make the several parts of the image cast on the second board successively pass through the hole in it, that I might observe to what places on the wall the second prism would refract them. And I saw by the variation of those places that the light, tending to that end of the image toward which the refraction of the first prism was made, did in the second prism suffer a refraction considerably greater than the light tending to the other end. And so the true cause of the length of that image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence were, according to their degrees of refrangibility, transmitted to divers parts of the wall.

(1671/1953:71–2)

The reader might have noticed that there is no mention of colour in the description of this experiment; instead, Newton has presented an experiment and a conclusion about the nature of light. What, then, is the connection between colour and the differential refrangibility of light?

By rotating the prism at the window, Newton was able to move the oblong image of the spectrum up and down on the second board, so that different portions of the spectrum would fall on its aperture and pass through, where they would be refracted again by the second prism. Since both boards and the second prism were fixed in place, the angle of incidence was constant for any beam of light striking the second prism. What Newton discovered was that despite this equality, different portions of the spectrum were refracted to different f erent degrees by the second prism. Light that came from the violet end of the spectrum was refracted the most, whereas light that came from the red end of the spectrum was refracted the least. Furthermore, the colours of the beams remained unchanged when they were passed through the second prism. Newton’s conclusion, therefore, was not merely that light ‘consists of rays differently refrangible’, but that the colours we experience correspond to different degrees of refrangibility. In his words: ‘To the same degree of refrangibility ever belongs the same color, and to the same color ever belongs the same degree of refrangibility’ (1671/1953:74).

Before Newton no one had held that colours correspond to the component rays of a colourless beam of light, or that these rays are specified by their differential refrangibility (Sepper 1988:109). Instead, the dominant theories of the time held that colours are produced as a result of the contact of light with some reflecting or refracting body (Sabra 1967/1981:294–5; Guerlac 1986). This type of theory is called modificationism by historians: colours are thought to arise because of some modification of colourless light. When light passes through a prism, for example, it becomes coloured on account of refraction. But the colours do not exist in the light prior to refraction; they are instead produced by the prism. For example, Sabra describes Hooke as having held that

the pulse of white light could be imagined as the resultant of a large number of ‘vibrations’ each of which when differentiated would produce a given colour. This would imply that white light is compounded only in a mathematical sense, and prismatic analysis would be understood as a process in which colours are manufactured out of the physically simple and undifferentiated white pulse.

(Sabra 1967/1981:233–4)

As Sepper notes, these modificationist theories ‘could separate the geometric from the chromatic problem: The refraction might be calculated according to the sine law, and the colors explained according to some notion of modification’ (Sepper 1988:124). In other words, for these theories there was no intrinsic connection between the refraction of light and colours. The refraction of light through a prism was to be calculated according to the laws of optics, in particular those based on Snell’s law of sines. Colours, however, were to be explained by the ways in which the prism changes the light that passes through it. For such theories there is no intrinsic correspondence, pre-existing in the light, between the degrees to which light can be refracted and dispositions to exhibit colours. Geometry and colour have not yet been united.

Newton’s experimentum crucis linked these two problems. At the outset of his ‘New theory about light and colors’, Newton presented himself as concerned with the chromatic problem, i.e., with ‘the celebrated phenomena of colors’. But despite this claim, he quickly turned to the geometrical problem of refraction: why, for a given position of the prism (minimum deviation), and assuming that the light from the sun is everywhere equally refrangible, is the image of the spectrum elongated? The experimentum crucis led Newton to conclude that the rays from the sun are not equally, but differentially refrangible: the rays preserve their respective degrees of refrangibility upon being refracted through the second prism. Newton then concluded, apparently on geometrical grounds alone, that sunlight consists of differentially refrangible rays, and so is not homogeneous. (The qualification ‘apparently’ is added because, as we shall see later, chromatic considerations in fact enter into the specification of what seems purely geometrical.) But the experimentum crucis also showed that the rays preserve their colour upon being refracted a second time; therefore, the same colour is always attached to the same index of refrangibility. Newton thus drew the further conclusion that colours are not modifications or ‘qualifications of light… but original and connate properties, which in divers rays are divers’ (1671/1953:74). These differentially refrangible rays with their colour-producing properties exist unseen in the white light of the sun; the refractive properties of the prism merely render this fact visible.

Here geometry and colour are united, or better yet, intertwined. For perhaps the first time there is held to be an intrinsic correspondence between properties of light and colour, one that is specified mathematically according to a geometrical index of refraction. In Sepper’s words: ‘Newton’s theory combined the geometric and chromatic problems to explain both refraction and color according to a single principle, differential refrangibility, executed as rigorously as geometry allowed’ (Sepper 1988:110).

This combination of the mathematical and the chromatic was to have decisive and profound consequences for both the science and philosophy of colour. To appreciate these consequences, however, we must realize that Newton’s argument is not conclusive. Its basic structure is easy to summarize.³

1. There are two and only two possibilities: either it is the case that
   
   • the colours of the spectrum are manufactured by the prism
   • the colours have been with the rays from their origin.
2. If (a), then
   
   • white light is homogeneous and colours are modifications, etc.
3. The crucial experiment shows that an isolated beam of coloured light, when refracted through a second prism, does not undergo any modification in colour or degree of refrangibility.
4. Therefore (c) is false.
5. Therefore (a) is false.
6. Therefore (b) is true.

It should be apparent that (4) does not follow from (3), i.e., that the crucial experiment is not sufficient to establish the falsity of (c) and hence the truth of (b). What (3) demonstrates is that light which emerges from the first prism behaves differently from direct sunlight: light from the sun is altered in its refraction by the first prism, but is not altered again in its refraction through the second prism. This demonstration leaves open several possibilities: for example, it still remains possib...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• FIGURES
• PREFACE
• ACKNOWLEDGEMENTS
• 1: THE RECEIVED VIEW
• 2: COLOUR VISION: RECENT THEORIES AND RESULTS
• 3: NATURALISTIC ONTOLOGIES
• 4: THE COMPARATIVE ARGUMENT
• 5: THE ECOLOGICAL VIEW
• 6: VISUAL EXPERIENCE AND THE ECOLOGICAL VIEW
• NOTES
• REFERENCES