"[V]ariations of tint occur: 1) Because colours are introcepted by varying and irregular strengths of light and shade. ...(2) because the colours blent vary in fullness and in effectiveness. ...(3) because they are blent in different proportions. ...(4) Difference of hue may also depend on the relative brightness and lustre or dimness and dullness of the blend. Lustre is simply continuity and density of light... (5) some objects change their colour and assume a variety of hues when polished by rubbing or other means, like silver, gold, copper, and iron, when they are polished...(6) In the case of objects burning, dissolving, or melting in the fire, we find that those have the greatest variety which are dark in colour... (7) Apart from these cases, variety of hue is characteristic of all dark smooth objects, such as water, clouds, and birds plumage. ...(8) Lastly, we never see a colour in absolute purity: it is always blent, if not with another colour, then with rays of light or with shadows, and so it assumes a new tint."