Goethe's Theory of Colours · Johann Wolfgang von Goethe
204.
If we attentively examine these opposite coloured edges, we find that they only appear in the direction of the apparent change of place. A round figure leaves us in some degree uncertain as to this: a quadrangular figure removes all doubt. The quadrangular figure _a_,[6] moved in the direction _a b_ or _a d_ exhibits no colour on the sides which are parallel with the direction in which it moves: on the other hand, if moved in the direction _a c_, parallel with its diagonal, all the edges of the figure appear coloured.[7]
Text from Project Gutenberg, public domain in the United States.