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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.

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