Having in this way found the clue to the true genesis of the spectrum, Goethe could not fail to notice that it called for another - a 'negative' spectrum, its polar opposite - to make the half into a whole. For he who has once learnt that light and darkness are two equally essential factors in the birth of colour, and that the opposing of two borders of darkness so as to enclose a light is a 'derived' (abgeleitet) experimental arrangement, is naturally free to alter the arrangement and to supplement it by reversing the order of the two borders, thus letting two lights enclose a darkness between them.