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.