The importance which Goethe himself saw in this aspect of the optical problem is shown by the place he gave it in the didactic part of his Farbenlehre. The first three chapters, after the Introduction, are called 'Physiological Colours', 'Physical Colours', and 'Chemical Colours'. In the first chapter, Goethe summarizes a group of phenomena which science calls 'subjective' colours, since their origin is traced to events within the organ of sight. The next chapter deals with an actual physics of colour - that is, with the appearance of colours in external space as a result of the refraction, diffraction and polarization of light. The third chapter treats of material colours in relation to chemical and other influences. After two chapters which need not concern us here comes the sixth and last chapter, entitled 'Physical-Moral Effect of Colour' ('Sinnlich-sittliche Wirkung der Farben'), which crowns the whole. There, for the first time in the history of modern science, a bridge is built between Physics, Aesthetics and Ethics. We remember it was with this aim in view that Goethe had embarked upon his search for the solution of the problem of colour.