*

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.

If one exposes an arrangement like this to the action of the prism,
whose position has remained unchanged, colours appear on each of the
two edges, as before, but in reverse order (Fig. iii). The spectral
phenomenon now begins at one side with light blue and passes into
indigo and violet, with uncoloured darkness in the centre. From this
darkness it emerges through red and passes through orange to yellow at
the other end.

Again, where the two interior colour-cones merge, there an additional
colour appears. Like green, it is of a neutral character, but at the
same time its quality is opposite to that of green. In Newtonian
optics, which assumes colour to be derived from light only, this colour
has naturally no existence. Yet in an optics which has learnt to reckon
with both darkness and light as generators of colour, the complete
spectrum phenomenon includes this colour equally with green. For lack
of an existing proper name for it, Goethe termed it 'pure red' (since
it was free from both the blue tinge of the mauve, and the yellow tinge
of the red end of the ordinary spectrum), or 'peach-blossom'