50development of the basic ideas of the duplicity theory

an achromatic light in the same area, the achromatic light appears greenish. In this case, the green colour observed was explained by Hering in a straightforward manner by the assumption that the sensitivity of the ‘red’ dissimilation process compared to that of the ‘green’ assimilation process was reduced in the stimulated area during the pre-stimulation period.

4.6  Contributions of Hering

It can be seen that Hering agreed with Goethe that opponency is a fundamental characteristic of colour phenomena, but in addition he also postulated an opponency of underlying material events. This clearly opposes the suggestion of the Young-Helmholtz colour theory (1896) that the nervous pathways, from receptor to brain, are like isolated telegraph wires with no interaction between different neural processes.

With hindsight, we can see that Hering pointed in the right direction. Indeed, his ingenious ideas deserve to be ranked as the fifth major paradigm shift in vision research. Yet, his phenomenological approach has two serious weaknesses. Firstly, the phenomenological report gives conclusive information only on the relative values of the underlying material processes. In the case of the successive contrast experiment, for instance, where the red pre-stimulation produced a green after-effect upon test stimulation, Hering could reasonably presume that the sensitivity for dissimilation relative to that of assimilation was reduced in the red-green see-substance. But even if this presumption is accepted, he was not in a position to decide whether the sensitivity for assimilation in the test area increases or remains constant. The sensitivity might even decrease, although less so than for dissimilation (Hering, 1878, p. 86).

Secondly, presupposing that the psychophysical maxim of Mach (1865) is valid, one may draw legitimate conclusions about material processes directly underlying consciousness, but not about visual processes prior to the final material process. This limitation of the approach is clearly revealed when the conditions are arranged so

the goethe tradition: the phenomenological approach 51

that test stimulation of rods and cones initiates the same achromatic colour. Since the subject cannot discriminate between the achromatic colours obtained under photopic and scotopic conditions, it follows from the maxim of Mach that the underlying material events are the same. Hence, the researcher may be misled into neglecting the important retinal differences involved.

Of course, the finding that the rod and cone receptor systems may give rise to the same achromatic colour sensation was also a major challenge for the Young-Helmholtz trichromatic colour theory. Here, the white sensation was assumed to result from an equal stimulation of the three primary cone receptor systems, while the rod system did not have any significant role to play (see Helmholtz, 1867; von Kries, 1911).

These serious shortcomings of the Hering and Young-­Helmholtz colour theories pointed to the need for a more comprehensive theory of colour vision where all the three major traditions were incorporated (i.e. the Newton, Schultze and Goethe traditions). This lead was followed by several research workers in the early 1900s. The most detailed and comprehensive theories were provided by Armin Tschermak (1902, 1929) and George Elias Müller (1896, 1897, 1923, 1930), both rooted in the Goethe tradition.

5  The colour theories of Armin

Tschermak and George Elias Müller

5.1  The colour theory of Tschermak

In developing his own colour theory, Tschermak (1902, 1929) made a critical evaluation of the colour theories of Young-Helmholtz, Schultze and Hering. With regard to Young-Helmholtz’s colour theory, he was severely critical. Thus, he asserted that the basic assumption of three independent, primary colour-related processes postulated by the trichromatic theory could not be reconciled with the phenomenological analysis of colour sensation that revealed six qualitatively different unitary sensations: red, yellow, green, blue, white and black. It would, for example, be impossible to give an adequate explanation of the uncompounded yellow-related material process by greenand red-related processes, or the uncompounded white-related process by red-, greenand violet-related processes. Also, in opposition to the trichromatic colour theory, experiments on colour mixture, colour induction and colour contrast clearly revealed opponent interaction processes going on in the visual system.

Finally, in accord with von Kries (1911), Tschermak (1902, 1929) pointed out that the basic assumption of the trichromatic colour theory, that white sensation was generated when the three different types of cone receptors were activated to about the same degree, was seriously challenged by the fact that colourless sensation could also be observed in scotopic vision where only rod receptors were known to function.

With regard to Schultze’s duplicity theory, on the other hand, he found the evidence strongly in favour of its basic assumptions that cones functioned in day vision giving rise to both achromatic and chromatic sensations, and that rods functioned in night vision, giving

the colour theories of a. tschermak and g. e. müller 53

rise to achromatic sensation only. He also accepted the basic assumption of Hering’s opponent colour theory that colour vision rested upon independent, opponent ‘red-green’, ‘yellow-blue’ and ‘whiteblack’ material processes. On the basis of his evaluation, he came to the conclusion that there were five different kinds of cone receptor systems in the retina: an achromatic-, a red-, a yellow-, a greenand a blue-related system, in addition to the achromatic rod system.

Yet, his most original theoretical contribution was his suggestion that rhodopsin was situated not only in rods but also in small amounts in cones (Tschermak, 1929, p. 576). He found supporting evidence in the fact that several research workers had obtained the Purkinje phenomenon at the central fovea (Tschermak, 1902, p. 720). That some researchers (see e.g. von Kries, 1929) were unable to repeat this observation, he explained by the suggestion that the Purkinje phenomenon at the fovea was less pronounced than that obtained extrafoveally.

The suggestion of Tschermak that cones also contained rhodopsin raised the important question of how stimulation of rhodopsin might influence chromatic colour vision – a question discussed at length by G. E. Müller.

5.2  The duplicity theory of G. E. Müller

Along with von Kries, G. E. Müller was generally considered to be the leading authority in vision research in the 1920s. His colour theory was much more detailed and comprehensive than that of Tschermak (see G. E. Müller, 1896, 1897, 1923, 1930). It incorporated the basic knowledge of rod and cone functions accumulated up to 1930 and may be seen to represent the end of the first phase in the development­ of the duplicity theory, integrating evidence from the Newton, Schultze and Goethe traditions.

His theory, though, is highly speculative. Thus, in sharp contrast to the cautious theory construction of von Kries (1911), it gives detailed descriptions of colour processes in the visual pathway based on an almost complete lack of factual knowledge of the underlying neurophysiological processes. With hindsight, one may ask whether