There seems to be no suitable word in English for rendering the term trübe in the sense in which Goethe used it to denote the optical resistance of a more or less transparent medium. The following remarks of Goethe’s, reported by his secretary Riemer, will give the reader a picture of what Goethe meant by this term, clear enough to allow us to use the German word. Goethe’s explanation certainly shows how inadequate it is to translate trübe by ‘cloudy’ or ‘semi-opaque’ as commentators have done. ‘Light and Dark have a common field, a space, a vacuum in which they are seen to appear. This space is the realm of the transparent. Just as the different colours are related to Light and Dark as their creative causes, so is their corporeal part, their medium, Trübe, related to the transparent. The first diminution of the transparent, i.e. the first slightest filling of space, the first disposition, as it were, to the corporeal, i.e. the non-transparent – this is Trübe.‘⁸