He saw that such an effect was presented to his eye when he turned his gaze on the one hand to the blue sky, and on the other to the yellowish luminous sun. Where we see the blue of the heavens, there, spread out before our eyes, is universal space, which as such is dark. Why it does not appear dark by day as well as by night is because we see it through the sun-illumined atmosphere. The opposite role is played by the atmosphere when we look through it to the sun. In the first instance it acts as a lightening, in the second as a darkening, medium. 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.

The ur-phenomenon having once been discovered in the heavens, could then easily be found elsewhere in nature on a large or small scale-as, for instance, in the blue of distant hills when the air is sufficiently opaque, or in the colour of the colourless, slightly milky opal which looks a deep blue when one sees it against a dark background, and a reddish yellow when one holds it against the light. The same phenomenon may be produced artificially through the clouding of glass with suitable substances, as one finds in various glass handicraft objects. The aesthetic effect is due to the treated glass being so fashioned as to present continually changing angles to the light, when both colour-poles and all the intermediate phases appear simultaneously. It is also possible to produce the ur-phenomenon experimentally by placing a glass jug filled with water before a black background, illuminating the jug from the side, and gradually clouding the water by the admixture of suitable substances. Whilst the brightness appearing in the direction of the light goes over from yellow and orange to an increasingly red shade, the darkness of the black background brightens to blue, which increases and passes over to a milky white.

It had already become clear to Goethe in Italy that all colour-experience is based on a polarity, which he found expressed by painters as the contrast between 'cold' and 'warm' colours. Now that the coming-into-being of the blue of the sky and of the yellow of the sun had shown themselves to him as two processes of opposite character, he recognized in them the objective reason why both colours are subjectively experienced by us as opposites. 'Blue is illumined darkness - yellow is darkened light' - thus could he assert the urphenomenon, while he expressed the relation to Light of colours in their totality by saying: 'Colours are Deeds and Sufferings of Light.'

With this, Goethe had taken the first decisive step towards his goal - the tracing of man's aesthetic experience to objective facts of nature.

If we use the expressions of preceding chapters, we can say that Goethe, in observing the coloured ur-phenomenon, had succeeded in finding how from the primary polarity, Light-Dark, the opposition of the yellow and blue colours arises as a secondary polarity. For such an interplay of light and darkness, the existence of the air was seen to be a necessary condition, representing in the one case a lightening, in the other, a darkening element. That it was able to play this double role arose from its being on the one hand pervious to light, while yet possessing a certain substantial density. For a medium of such a nature Goethe coined the expression trübes Medium.

There seems to be no suitable word in English for rendering the term trübe in the sense in which Goethe used it to denote the optical resistance of a more or less transparent medium. The following remarks of Goethe's, reported by his secretary Riemer, will give the reader a picture of what Goethe meant by this term, clear enough to allow us to use the German word. Goethe's explanation certainly shows how inadequate it is to translate trübe by 'cloudy' or 'semi-opaque' as commentators have done. 'Light and Dark have a common field, a space, a vacuum in which they are seen to appear. This space is the realm of the transparent. Just as the different colours are related to Light and Dark as their creative causes, so is their corporeal part, their medium, Trübe, related to the transparent. The first diminution of the transparent, i.e. the first slightest filling of space, the first disposition, as it were, to the corporeal, i.e. the non-transparent - this is Trübe.'⁸

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.

We must remember that on first looking through the prism Goethe had immediately recognized that the appearance of colour is always dependent on the existence of a boundary between light and darkness - in other words, that it is a border phenomenon. What colours appear on such a border depends on the position of light and darkness in relation to the base of the prism. If the lighter part is nearer to the base, then blue and violet tints are seen at the border, and with the reverse position tints of yellow and red (Plate B, Fig. i). Along this path of study Goethe found no reason for regarding the spectrum-phenomenon as complete only when both kinds of border-phenomena appear simultaneously (let alone when - as a result of the smallness of the aperture through which the light meets the prism - the two edges lie so close that a continuous band of colour arises). Hence we find Goethe - unlike Newton - treating the two ends of the spectrum as two separate phenomena.

In this way, the spectrum phenomenon gave Goethe confirmation that he had succeeded in expressing in a generally valid form the law of the origin of the blue and the yellow colours, as he had read it from the heavens. For in the spectrum, too, where the colour blue appears, there he saw darkness being lightened by a shifting of the image of the border between light and dark in the direction of darkness; where yellow appears, he saw light being darkened by a shifting of the image in the direction of light. (See the arrow in Fig. i.)

In the colours adjoining these - indigo and violet on the blue side, orange and red on the yellow side - Goethe recognized 'heightened' modifications of blue and yellow. Thus he had learnt from the macro-telluric realm that with decreasing density of the corporeal medium, the blue sky takes on ever deeper tones, while with increasing density of the medium, the yellow of the sunlight passes over into orange and finally red. Prismatic phenomenon and macrotelluric phenomenon were seen to correspond in this direction, too.