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.