Hering was more concerned with the introspective aspects of colours. He also spent considerable time investigating the eye’s perception of three-dimensional space. His work on colour refers to the problem of yellow in the three-colour-system, for example. According to Helmholtz, yellow was of necessity produced from a mixture of red and green, but this — so Hering realised — was not in line with human experience. The sensation of yellow is elementary, and not traceable to a mixture. Hering states that there are, in addition to black and white, four colours which «can occur without a tinge of another colour» and recommends that «each visual perception» can be seen as a «mixture of the six basic sensations» which oppose each other and thus interact. (Detailed text)
In the middle of the 19th century, it was accepted that only three variables, in other words three receptors, are required to explain the colour mixing which formed the basis of experiments carried out by both James Maxwell in 1867 and Hermann von Helmholtz in 1859. Modern physiologists can confirm only that three types of molecule (photo-receptors) exist, and that each type is particularly sensitive to either short, medium and long waves. Although this observation can help explain why a few wavebands of incident light cannot be distinguished from others and thus why many mixtures result in the same colours, we are nevertheless unable to explain those colour-hues which we can see.
It was Helmholtz who had assumed that there must be three receptors, each directly signaling a definite colour-hue, and he consequently named these receptors «blue», «green» and «red» in the belief that the blue receptor, for example, produced the sensation of blue and so on. He was, of course, aware that the spectral sensitivity of the receptors had to overlap, so that each wavelength could give rise to varying colour relationships (and another perception). Between 1872 and 1874, the physiologist Ewald Hering (1834-1918) had delivered «six communications» entitled On the Theory of Sensibility to Light at the Academy of Sciences in Vienna — privately published in 1878 — in which Hering opposed the Helmholtz view of the phenomenon of colours. (From 1905 onwards, he published his Principles of the Theory of Sensitivity to Light. These appeared in four instalments of which, incidentally, the third was broken off in mid-sentence, leaving the reader to wait years for the full meaning!)
Although he also spent considerable time investigating the eye’s perception of three-dimensional space, Hering was more concerned with the introspective aspects of colours. His work on colour refers to the problem ofyellow in the three-colour system, for example. According to Helmholtz, yellow was of necessity produced from a mixture of red and green, but this — so Hering realised — was not in line with human experience. The sensation of yellow is elementary, and not traceable to a mixture. Hering further states that mixtures of red and green never occur, but eliminate each other. A red-green is simply inconceivable.
Hering therefore concludes that there are not three but four elementary colour sensations or psychological primaries which code our perception by means of so-called opponent-processes. In 1878, Hering wrote: «Yellow can have a red or green tinge, but not a blue one; blue can have only either a red or a green tinge, and red only either a yellow or a blue one. The four colours can with complete correctness therefore be described as simple or basic colours, as Leonardo da Vinci has already done. Language, too, has simple descriptions of them, and not expressions borrowed from coloured natural bodies.»
In the case of opponent-colours, which account for all colour-hues of the visible spectrum, Hering also speaks of «antagonistic types of light…which together produce white». That means «they do not complement each other to form white; they merely allow white to occur as pure because, as antagonists, they render each other’s effect impossible». White was for him «a sensation of its own nature, in the same way as black, red, green, yellow or blue». Hering therefore additionally proposed a white-black opponent-process in order to cater for brightness. There are thus six basic colour-hues in all.
Hering expressively distanced his Theory of the Sensitivity of Light from the world of physics. To Hering, the claim that red and green or blue and yellow together give white would «only make sense if red and green are understood as oscillations of the ether, and not red and green sensations».
Experiments using test-subjects to describe an impression of coloured light now confirm Hering’s opponent-theory convincingly; four expressions — red, green, yellow and blue — were available to the early pioneers, and they were able to describe each colour using suitable combinations of these terms. There are in fact four (and not three) fundamental colour-hues (the neurophysiological proof has been available since 1966, but we shall look into this elsewhere) and these are placed opposite each other in Hering’s system, which is a circle of opponent rings and ellipses. This has been reproduced here with its four basic colours of yellow (Y), red (R), blue (B) and green (G) arranged at right angles to one another. The broken lines refer to mixtures at a ratio of 50:50: yellow-red (YR), red-blue (RB), blue-green (BG) and green-yellow (GY). To the lower right, we have stretched the opponent-circle out into a strip.
Hering’s order of colours, which he described as «the natural system of colour sensations», forms the basis of a system nowadays known by the three letters NCS, i.e. the «Natural Colour System». The succession of the colour-circle shows the position of the four «elementary» colours, and the proportions with which any two elementary colours can form mixtures.
Hering’s opponent-theory was not accepted, being criticised chiefly by students of Helmholtz who argued that Hering’s proposal only made sense if two different processes existed within the nervous system: namely: stimulating and moderating processes. In Hering’s day, the kind of knowledge which we now take for granted had still to be acquired and subjected to proof. Nevertheless, our perception of colours is no clearer, but we shouldn’t allow that to spoil our enjoyment of them.
Date: In 1878, the physiologist Ewald Hering published his On the Theory of Sensibility to Light in Vienna, which opposed the purely phenomenal or physical understanding of colours.
Country of origin: Austria
Basic colours: Blue, red, yellow and blue
Form: Blue, red, yellow and blue
Related systems: Pythagoras, Aristoteles, Platon — Maxwell — Helmholtz — Blanc — Höfler — Boring — Birren — Johansson — Hesselgren — N.C.S. — C.I.E.L.A.B. — Albert-Vanel
Bibliography: E. Hering, «Zur Lehre vom Lichtsinn» (illustration), Vienna 1878; G. A. «Agoston Color Theory and Its Application in Art and Design», Heidelberg 1979; S. Hesselgren, «Why Colour Order Systems?», Color Research and Application 9, pp. 220 – 226 (1984).