seen at night but always in the neighbourhood of the sun, either as it
is rising or setting but more commonly towards sunset. They have
scarcely ever appeared when the sun was on the meridian, though this
once happened in Bosporus where two mock suns rose with the sun and
followed it all through the day till sunset.
These are the facts about each of these phenomena: the cause of them
all is the same, for they are all reflections. But they are
different varieties, and are distinguished by the surface from which
and the way in which the reflection to the sun or some other bright
object takes place.
The rainbow is seen by day, and it was formerly thought that it
never appeared by night as a moon rainbow. This opinion was due to the
rarity of the occurrence: it was not observed, for though it does
happen it does so rarely. The reason is that the colours are not so
easy to see in the dark and that many other conditions must
coincide, and all that in a single day in the month. For if there is
to be one it must be at full moon, and then as the moon is either
rising or setting. So we have only met with two instances of a moon
rainbow in more than fifty years.
We must accept from the theory of optics the fact that sight is
reflected from air and any object with a smooth surface just as it
is from water; also that in some mirrors the forms of things are
reflected, in others only their colours. Of the latter kind are
those mirrors which are so small as to be indivisible for sense. It is
impossible that the figure of a thing should be reflected in them, for
if it is the mirror will be sensibly divisible since divisibility is
involved in the notion of figure. But since something must be
reflected in them and figure cannot be, it remains that colour alone
should be reflected. The colour of a bright object sometimes appears
bright in the reflection, but it sometimes, either owing to the
admixture of the colour of the mirror or to weakness of sight, gives
rise to the appearance of another colour.