34development of the basic ideas of the duplicity theory

conditions, the rod receptor system was not equally activated in the test and comparison fields.

König’s third thesis is the most original one. It suggests, in opposition to both Schultze (1866) and Parinaud (1881, 1885), that rods may play an important role in colour vision. Previously, Ebbinghaus (1893) had speculated that the photopigments of rods could be identified as the yellow-blue substrate postulated by Hering (1878). Thus, the action of light on rhodopsin and visual yellow (a photoproduct of rhodopsin) was supposed to give rise to the sensation of yellow and blue, respectively. In agreement with Ebbinghaus, König (1894) assumed that the formation of visual yellow was necessary in order to obtain the primary blue sensation. Consequently, he held that the rod-free fovea was blue-blind. In opposition to Ebbinghaus, however, he suggested that activation of rhodopsin generated achromatic instead of yellow sensations – a suggestion based on his finding that light appeared achromatic, not yellow, under scotopic conditions where rhodopsin was responsible for vision.

In order to reconcile his assumption that visual yellow and rhodopsin situated in the same receptor generated two ­qualitatively different sensations with the specific fibre-energy doctrine, König (1894, p. 591) referred to complex processes originating in thesensorium. Other ways to explain this would have been to accept the ad hoc assumption of Helmholtz (1867) that different qualities may be transmitted through the same fibre or, alternatively, to presume that two different fibres, one subserving achromatic sensation, the other blue, were connected to each rod.

3.12  The duplicity theory of von Kries

In contrast to Schultze, Boll, Kühne, Parinaud and König, von Kries (1894, 1896, 1911, 1929) did not provide compelling new evidence in support of the duplicity theory. Instead, he fulfilled the important role of promoting its general acceptance by presenting a comprehensive discussion of all available evidence in favour of the theory, including his own accurate experimental results (see von Kries, 1929).

the schultze tradition 35

Since von Kries was recognized as the leading authority on the duplicity theory, and the 1929 paper gives his final and most thorough defence of the theory, we will present this defence in some detail. The main evidence supporting the duplicity theory was discussed under three major headings:

1.  Lights that match in day vision may differ in twilight vision: the Purkinje phenomenon.

Here, he presented experimental results which showed that a colour match between two light patches, obtained extrafoveally in a lightadapted state, may break down when the intensity of the two fields is lowered equally and the eye becomes dark adapted. Indeed, he showed that a colour match obtained in a light-adapted state at mesopic luminance levels may break down by dark adaptation without any change in light intensity.

As noted above, both of these observations were opposed to Newton’s additivity law of colour mixture. With a suggestion similar to that of König (1894), von Kries explained the apparent additivity failure by the assumption that there were two different modes of vision, a chromatic day vision and an achromatic twilight vision, and that, as the mode of vision gradually changed from day vision to twilight vision, the colour field with the strongest twilight activation would gradually become relatively more desaturated and bright.

Obviously, this explanation implies that the two modes of vision may function simultaneously in an overlapping intensity range (the so-called mesopic intensity range), and that an increase of rod activity in a test field may reduce the saturation and increase the brightness of the test colour – all in agreement with Schultze (1866), Parinaud (1884b, 1885) and König (1894).

Strong support of the duplicity theory was also found in the observation that the breakdown of colour matches could be obtained in all parts of the extrafoveal retina, from the parafovea to the extreme periphery, but not in the central fovea where only cones were operating, and that only achromatic colours could be seen in twilight vision.

36development of the basic ideas of the duplicity theory

2.  Anatomical interpretation of the theory. Cones and Rods. Uniqueness of the fovea. Rhodopsin.

Under this second heading, von Kries referred to a number of experiments which demonstrated the complete absence of thePurkinje shift in the rod-free foveal region. The evidence he found supported the suggestion that the cone receptors were the ­anatomical substrate of day vision, while the rods were the anatomical substrate of twilight vision. An even stronger argument in favour of this suggestion was the close relationship found between the spectral sensitivity of twilight vision and the degree of ­decomposition of rhodopsin with wavelength. ­Surprisingly, von Kries did not refer to König (1894), but to Trendelenburg, who much later obtained results similar to those of König (see von Kries, 1929, p. 691).

Lastly, the corresponding time between regeneration of rhodopsin and the sensitivity increase during dark adaptation measured extrafoveally (both processes take about 30 min to approach the final level) were mentioned as important evidence supporting the duplicity theory.

3.  Isolation of twilight vision. Congenital, total colour-blindness. Nyctalopia. On comparative anatomy.

Here, under this third heading, von Kries presented experiments which showed that the spectral luminosity function in twilight vision was approximately the same and remained essentially invariant within the scotopic intensity range when measured in trichromats, dichromats and rod monochromats. In Fig. 3.1 the spectral sensitivity of rods and cones obtained at 30° extrafoveally are shown. As can be seen, the sensitivity difference between rods and cones is large in the short-wave and small in the long-wave region.

Finally, Schultze’s observation that the relative number of rods to cones differed in the retina of diurnal and nocturnal animals, and the investigation of ‘l’héméralopie’ by Parinaud (1881), were found to support the duplicity theory.