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5 The difference in character of the various parts of the spectrum, as described above, comes out particularly impressively if for capturing the colour-phenomenon one uses instead of a flat white surface, a clear crystal of not too small size, or else a cluster of crystals moving it slowly along the coloured band from one end to the other. Towards a New Cosmosophy
In spite of all that he achieved, however, he never reached a real solution of the riddle of the colour-phenomenon produced when light passes through a transparent body of prismatic shape.
It is characteristic of Goethe's whole mode of procedure that he at once changed the question, 'What is colour? into the question, 'How does colour arise? It was equally characteristic that he did not, as Newton did, shut himself into a darkened room, so as to get hold of the colour-phenomenon by means of an artificially set-up apparatus.
As already mentioned, Goethe's first inspection of the colour-phenomenon produced by the prism had shown him that the phenomenon depended on the presence of a boundary between light and darkness. Newton's attempt to explain the spectrum out of light alone appeared to him, therefore, as an inadmissible setting aside of one of the two necessary conditions.
Accordingly, when the optical density of the air changes as a result of its varying content of moisture, the colour-phenomenon undergoes an opposite change in each of the two cases. Whilst with increasing density of the air the blue of the sky brightens up and gradually passes over into white, the yellow of the sun gradually darkens and finally gives way to complete absence of light.
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