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

After Goethe had once determined from the macrotelluric phenomenon that
an interplay of light and darkness within Trübe was necessary for the
appearance of colour in space, he had no doubt that the prismatic
colours, too, could be understood only through the coming together of
all these three elements. It was now his task to examine in what way
the prism, by its being trübe, brings light and darkness, or, as he also
expressed it, light and shadow, into interplay, when they meet at a
boundary.

We must remember that on first looking through the prism Goethe had
immediately recognized that the appearance of colour is always
dependent on the existence of a boundary between light and darkness -
in other words, that it is a border phenomenon. What colours appear on