the goethe tradition: the phenomenological approach 45

philosophers (e.g. Empedocles and Plato), and Aristotle had suggested that colour sensations could be generated by an interaction between white and black (Goethe, 1810, pp. 524–539). In fact, a colour theory similar to that of Goethe had previously been presented by Lazarus Nuguet (Goethe, 1810, pp. 748–756). Also, a serious limitation of his theory was the complete neglect of the physical and physiological basis of colour vision.

Yet, his historical review of colour theories (Goethe, 1810, Historischer Teil, pp. 513–986), his many experiments, his descriptions and analyses of a great variety of colour phenomena, his insistence on a phenomenological analysis of colour vision in its own right, even including the emotional characteristics of colours, and his concept of opponency, made a profound impact on later research within colour vision. The comprehensive work of Goethe made it quite clear that the observational basis of Newton had to be expanded in order to reach a more general understanding of the colour phenomena.

4.4  The colour theory of Ewald Hering

Hering developed a new and ingenious colour theory by changing the basic presumptions of Goethe in two important ways (see Hering, 1878, 1964).

1. He proposed that the chromatic colours on the ­circumference of the colour circle should be ordered quantitatively. Thus, he presumed (a) that there are four primary hues, each of which reveals only one psychological quality: red, yellow, green or blue (this is in agreement with several other previous research workers, e.g. Aubert, 1865; Mach, 1865), and (b) that all the intermediate hues may be characterized by the relative distinctness of two of the primaries. Thus, all hues in the range between pure red and pure yellow, for example, may be characterized by the relative distinctness of red and yellow – the more similar the colour is to yellow, the less its similarity to red and vice versa. Accordingly, the colour series between pure red and pure yellow may be ordered quantitatively on the circumference­

of the circle by the ratio between the degree of similarity to red and

the goethe tradition: the phenomenological approach 47

could be expressed by six variables on whose relative proportion the different colour qualities would depend. From this finding and Mach’s psychophysical maxim, Hering (1878) concluded that the correlated material processes could be expressed similarly, and that the quality of a colour directly correlated with the ratio of the magnitude of its underlying material process to the sum of all colour-related material processes operating simultaneously in the stimulated area. This sum, in turn, correlated with the impressiveness of the colour sensation. The green colour quality in a green-grey colour, for example, could be specified as G/(G + W + B), that is, the magnitude of the material process underlying green relative to the sum of the material processes underlying green, white and black, while the impressiveness of the green-grey colour would correlate with the sum of the three different material processes (G + W + B) (Hering, 1878, pp. 82–85). His ­quantitative analysis and mathematical treatment were rooted in what he termed‘das allgemeine psychophysische Grundgesetz’ (‘the common basic psychophysical doctrine’), particularly the maxim that the purity, plainness or clearness of a sensation depends on the ratio of the weight of the sensation to the sum of the weights of all simultaneously activated sensations (see Hering, 1878, p. 84).

The phenomenological analysis of the visible spectrum also revealed that there was no series of red-green or yellow-blue intermediate­ hues. That is, no hue in the spectrum appeared simultaneously red and green, or yellow and blue (Hering, 1964, p. 50). This mutually exclusive character of red and green, and of yellow and blue was also clearly displayed by binary colour mixtures where the two chromatic components either cancelled each other out completely or to the extent that only the dominant one was observed. Furthermore, the phenomenological analysis of the white-grey-black colour series revealed that white and black colour sensations were also opposed to each other. Thus, Hering could point to the fact that when white increased, black decreased and vice versa.

From these phenomenological analyses he concluded that the material processes underlying red, yellow and white were ­antagonistic

48development of the basic ideas of the duplicity theory

to the material processes underlying, respectively, green, blue and black (Hering, 1878, pp. 70–126). Accordingly, he presumed that there were three pairs of opponent material processes: a ‘red-green’, a ‘yellow-blue’ and a ‘white-black’ pair, and that each pair had opposing metabolic dissimilation and assimilation processes in a ‘see-sub- stance’ (‘Sehsubstanz’). The red, yellow and white sensations were correlated with dissimilation processes that reduced the amount of the see-substance, while the green, blue and black sensations were correlated with assimilation processes that increased the amount of the see-substance. In the simplest version, the see-substance was assumed to consist of three different substances: a ‘red-green’, a ‘yellow-blue’ and a ‘white-black’ see-substance, where each could generate opposing dissimilation-assimilation processes independently of the activity of the other two. Hering did not locate these see-substances in the visual pathway. He simply suggested that waves of light in the ether generated chemical changes in the retina and that these changes triggered chemical processes in the optic nerve that eventually gave rise to chemical changes in the brain. Whether the see-substance was located only in the brain or also in the optic nerve and retina, he left undecided (Hering, 1878, pp. 70–75).

The ‘red-green’ see-substance could be activated by lights of all wavelengths, with the exception of pure yellow and pure blue, where the dissimilation and assimilation processes in the see-substance were assumed to be of equal magnitude, preventing any change in amount of substance. The same considerations also applied to the‘yellow-blue’ see-substance, except that dissimilation and assimilation in this substance were assumed to be of equal magnitude for pure red and pure green light.

The ‘white-black’ see-substance, on the other hand, was assumed to differ from the two chromatic-related see-substances in two important respects: (1) the dissimilation and assimilation processes going on in this substance in darkness were assumed to be relatively large, and (2) light from the spectrum was assumed to only produce dissimilation in the stimulated area.

the goethe tradition: the phenomenological approach 49

Interestingly, Hering speculated that if light could have activated the assimilation process in the ‘white-black’ see-substance, then the experience of black by such a light would have been much more intense than the most intense black we know of (Hering, 1878, pp. 88–89), and if a light stimulus could have produced equal ­dissimilation and assimilation in this substance alone, that light would have been invisible (Hering, 1878, p. 122).

A basic postulate in Hering’s colour theory was his assumption that all six primary material processes were always in operation in all parts of the see-substance, despite the antagonistic actions of the dissimilation and assimilation processes (Hering, 1878, p. 85, pp. 118–120, and p. 126). The question then arises as to why red and green, and yellow and blue colours are mutually exclusive at the conscious level. Hering attempted to resolve this apparent paradox with his suggestion that dissimilation and assimilation in the ‘redgreen’ and in the ‘yellow-blue’ see-substances had relatively low magnitudes and, hence, low impressiveness.

4.5  Experiments in support of Hering’s colour theory

Besides the phenomenological analysis of the visual spectrum and the binary colour-mixture experiments, Hering found experiments on simultaneous and successive contrast, and simultaneous andsuccessive induction, to be of particular importance in revealing information on the mechanisms of the underlying material processes (Hering,1878, pp. 95–103, 126–135). Thus, the experimental results he obtained could all easily be explained by the reasonable assumption that when one of the opponent processes was triggered by a light stimulus, the relative sensitivity of the antagonistic process increased with exposure time in the stimulated area and decreased in the neighbouring area.

As an illustration: when the eye, in a completely dark-adapted state, is pre-stimulated with a red light in a given retinal area (dissimilation increases) and, then, successively test-stimulated with