Depth in colour, a history of a chapter in physiologie optique amusante.

Two centuries ago, Goethe wrote in his Farbenlehre about blue as a receding colour, against yellow/red as a colour 'piercing into the organ'. Though any verification of these psychological attributes seems to be absent, this depth effect in colour still seems to be taken for granted in art history. In visual science, the completely different and far better documented colour stereoscopic effect, known as chromostereopsis, prevails. A historical account is given of the development of ideas about this chromostereopsis, often by amusing trials and errors. The original explanation by Bruecke in terms of chromatic aberration (in present-day literature usually indicated as transverse chromatic aberration) and a temporal off-axis position of the fovea still forms the basis of present-day understanding, even though Bruecke rejected his own explanation. It had to be complemented by an additional off-axis effect of the pupil, not in an anatomical sense, as Einthoven presumed but failed to find but in a functional sense by a non-centric Stiles-Crawford effect. Although a comprehensive individual experimental proof is still missing, the evidence available indicates that a satisfactory quantitative explanation has been reached.