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a reflection of sight to the sun.

So the rainbow always appears opposite the sun whereas the halo is

round it. They are both reflections, but the rainbow is

distinguished by the variety of its colours. The reflection in the one

case is from water which is dark and from a distance; in the other

from air which is nearer and lighter in colour. White light through

a dark medium or on a dark surface (it makes no difference) looks red.

We know how red the flame of green wood is: this is because so much

smoke is mixed with the bright white firelight: so, too, the sun

appears red through smoke and mist. That is why in the rainbow

reflection the outer circumference is red (the reflection being from

small particles of water), but not in the case of the halo. The

other colours shall be explained later. Again, a condensation of

this kind cannot persist in the neighbourhood of the sun: it must

either turn to rain or be dissolved, but opposite to the sun there

is an interval during which the water is formed. If there were not

this distinction haloes would be coloured like the rainbow. Actually

no complete or circular halo presents this colour, only small and

fragmentary appearances called 'rods'. But if a haze due to water or

any other dark substance formed there we should have had, as we

maintain, a complete rainbow like that which we do find lamps. A

rainbow appears round these in winter, generally with southerly winds.

Persons whose eyes are moist see it most clearly because their sight

is weak and easily reflected. It is due to the moistness of the air

and the soot which the flame gives off and which mixes with the air

and makes it a mirror, and to the blackness which that mirror

derives from the smoky nature of the soot. The light of the lamp

appears as a circle which is not white but purple. It shows the

colours of the rainbow; but because the sight that is reflected is too

weak and the mirror too dark, red is absent. The rainbow that is

seen when oars are raised out of the sea involves the same relative

positions as that in the sky, but its colour is more like that round

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