Colours seen in shadow will display more or less of their natural brilliancy in proportion as they are in fainter or deeper shadow. But if these same colours are situated in a well-lighted place, they will appear brighter in proportion as the light is more brilliant. THE ADVERSARY. The variety of colours in shadow must be as great as that of the colours in the objects in that shadow. THE ANSWER. Colours seen in shadow will display less variety in proportion as the shadows in which they lie are deeper. And evidence of this is to be had by looking from an open space into the doorways of dark and shadowy churches, where the pictures which are painted in various colours all look of uniform darkness. Hence at a considerable distance all the shadows of different colours will appear of the same darkness. It is the light side of an object in light and shade which shows the true colour. On the colours of the rainbow (287. 288). 287. Treat of the rainbow in the last book on Painting, but first write the book on colours produced by the mixture of other colours, so as to be able to prove by those painters' colours how the colours of the rainbow are produced. 288. WHETHER THE COLOURS OF THE RAINBOW ARE PRODUCED BY THE SUN. The colours of the rainbow are not produced by the sun, for they occur in many ways without the sunshine; as may be seen by holding a glass of water up to the eye; when, in the glass--where there are those minute bubbles always seen in coarse glass--each bubble, even though the sun does not fall on it, will produce on one side all the colours of the rainbow; as you may see by placing the glass between the day light and your eye in such a way as that it is close to the eye, while on one side the glass admits the [diffused] light of the atmosphere, and on the other side the shadow of the wall on one side of the window; either left or right, it matters not which. Then, by turning the glass round you will see these colours all round the bubbles in the glass &c. And the rest shall be said in its place. THAT THE EYE HAS NO PART IN PRODUCING THE COLOURS OF THE RAINBOW. In the experiment just described, the eye would seem to have some share in the colours of the rainbow, since these bubbles in the glass do not display the colours except through the medium of the eye. But, if you place the glass full of water on the window sill, in such a position as that the outer side is exposed to the sun's rays, you will see the same colours produced in the spot of light thrown through the glass and upon the floor, in a dark place, below the window; and as the eye is not here concerned in it, we may evidently, and with certainty pronounce that the eye has no share in producing them. OF THE COLOURS IN THE FEATHERS OF CERTAIN BIRDS.