XCVI
3) Morning twilight and evening twilight. The sun
shines before rising or after having set down the vapours con
tained in the air, and these reflect the light. Near the equator
almost no twilight, during summer in Europe almost through the
whole night, why?
4) The Rainbow, 398-401, is seen, if a raining cloud be
fore us and the sun behind us. In which case will it be a circle
or a semicircle? The rays of the sun are refracted in entering
the drops, reflected on the backside and refracted again when
going out. As the drops stand at different heights according
to the different magnitude of the angle of refraction, all the
colours of the spectrum will be seen. By second reflection inside
of the drops a secondary rainbow with reversed colours will
be caused.
5) The Halo is considered to be a phenomenon of inter
ference, produced by ice-crystals, driving in the air.
Remark. In many instances we have seen a wonderful harmony bet
ween the laws and phenomena of sound and those of light. As to the
colours we may say, they are the tones of light, tones produced by vibra
tions of an extraordinary velocity (see § 223). Sound-waves and light
waves show an admirable proportionality. The numbers of oscillation
for the seven colours of the spectrum are in a similar relation to each
other as the numbers of oscillation producing the tones of the musical
octave. Also the laws about the intensity of light and of sound are ana
logous. As to tones we know eight musical octaves, in the spectrum we
have only one octave of light, but in § 231 we have seen, that the world
of light hides riches our senses have not yet taken possession of.
On a higher stage of spiritual life they may be unveiled and rendered
accessible!
