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next stage is green, and a further degree of weakness gives violet. No

further change is visible, but three completes the series of colours

(as we find three does in most other things), and the change into

the rest is imperceptible to sense. Hence also the rainbow appears

with three colours; this is true of each of the two, but in a contrary

way. The outer band of the primary rainbow is red: for the largest

band reflects most sight to the sun, and the outer band is largest.

The middle band and the third go on the same principle. So if the

principles we laid down about the appearance of colours are true the

rainbow necessarily has three colours, and these three and no

others. The appearance of yellow is due to contrast, for the red is

whitened by its juxtaposition with green. We can see this from the

fact that the rainbow is purest when the cloud is blackest; and then

the red shows most yellow. (Yellow in the rainbow comes between red

and green.) So the whole of the red shows white by contrast with the

blackness of the cloud around: for it is white compared to the cloud

and the green. Again, when the rainbow is fading away and the red is

dissolving, the white cloud is brought into contact with the green and

becomes yellow. But the moon rainbow affords the best instance of this

colour contrast. It looks quite white: this is because it appears on

the dark cloud and at night. So, just as fire is intensified by

added fire, black beside black makes that which is in some degree

white look quite white. Bright dyes too show the effect of contrast.

In woven and embroidered stuffs the appearance of colours is

profoundly affected by their juxtaposition with one another (purple,

for instance, appears different on white and on black wool), and

also by differences of illumination. Thus embroiderers say that they

often make mistakes in their colours when they work by lamplight,

and use the wrong ones.

We have now shown why the rainbow has three colours and that these

are its only colours. The same cause explains the double rainbow and

the faintness of the colours in the outer one and their inverted

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