For the sky to appear blue by day a certain purity of the atmosphere is needed. The more veiled the atmosphere becomes the more the blue of the sky turns towards white; the purer and rarer the atmosphere, the deeper the blue, gradually approaching to black. To mountain climbers and those who fly at great heights it is a familiar experience to see the sky assume a deep indigo hue. There can be no doubt that at still higher altitudes the colour of the sky passes over into violet and ultimately into pure black. Thus in the case of blue the field of vision owes its darkening to a decrease in the resistance by which our visual ray is met in the optical medium. It is precisely the opposite with yellow. For here, as the density of the medium increases, the colour-effect grows darker by yellow darkening first to orange and then to red, until finally it passes over into complete darkness. This shows that our visual ray is subject to entirely different dynamic effects at the two poles of the colour-scale. At the blue pole, the lightness-effect springs from the resistant medium through which we gaze, a medium under the influence of gravity, while the darkness is provided by the anti-gravity quality of cosmic space, which as a 'negative' resistance exercises a suction on the eye's inner light. At the yellow pole it is just the reverse. Here, the resistant medium brings about a darkening of our field of vision, while the lightness-effect springs from a direct meeting of the eye with light, and so with the suctional effect of negative density. Our pursuit of the dynamic causes underlying our apperception of the two poles of the colour-scale has led us to a point where it becomes necessary to introduce certain new terms to enable us to go beyond Goethe's general distinction between Finsternis (darkness) and Licht (light). Following Goethe, we have so far used these two terms for what appears both in blue and yellow as the respective light and dark ingredients. This distinction cannot satisfy us any more. For through our last observations it has become clear that the Finsternis in blue and the Licht in yellow are opposites only in appearance, because they are both caused by Levity, and similarly that the lightening effect in blue and the darkening effect in yellow are both effected by Gravity. Therefore, to distinguish between what appertains to the primary polarity, Levity-Gravity, on the one hand, and their visible effects in the secondary polarity of the colours, on the other, we shall henceforth reserve the term darkness and, with it, lightness for instances where the perceptible components of the respective colours are concerned, while speaking of Dark and Light where reference is made to the generating primary polarity. *