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concomitants, and to assign their causes.

Sight is reflected from all smooth surfaces, such as are air and

water among others. Air must be condensed if it is to act as a mirror,

though it often gives a reflection even uncondensed when the sight

is weak. Such was the case of a man whose sight was faint and

indistinct. He always saw an image in front of him and facing him as

he walked. This was because his sight was reflected back to him. Its

morbid condition made it so weak and delicate that the air close by

acted as a mirror, just as distant and condensed air normally does,

and his sight could not push it back. So promontories in the sea

'loom' when there is a south-east wind, and everything seems bigger,

and in a mist, too, things seem bigger: so, too, the sun and the stars

seem bigger when rising and setting than on the meridian. But things

are best reflected from water, and even in process of formation it

is a better mirror than air, for each of the particles, the union of

which constitutes a raindrop, is necessarily a better mirror than

mist. Now it is obvious and has already been stated that a mirror of

this kind renders the colour of an object only, but not its shape.

Hence it follows that when it is on the point of raining and the air

in the clouds is in process of forming into raindrops but the rain

is not yet actually there, if the sun is opposite, or any other object

bright enough to make the cloud a mirror and cause the sight to be

reflected to the object then the reflection must render the colour

of the object without its shape. Since each of the mirrors is so small

as to be invisible and what we see is the continuous magnitude made up

of them all, the reflection necessarily gives us a continuous

magnitude made up of one colour; each of the mirrors contributing

the same colour to the whole. We may deduce that since these

conditions are realizable there will be an appearance due to

reflection whenever the sun and the cloud are related in the way

described and we are between them. But these are just the conditions

under which the rainbow appears. So it is clear that the rainbow is

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