that no other conditions contributed to the effect.

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