those that have a body, and are opake, it matters not what ground they have, and a white one is of no use. This is exemplified by painted glasses; when placed between the eye and clear air, they exhibit most excellent and beautiful colours, which is not the case, when they have thick air, or some opake body behind them.
Chap. CCXXXIII.—Of transparent Colours.
When a transparent colour is laid upon another of a different nature, it produces a mixed colour, different from either of the simple ones which compose it. This is observed in the smoke coming out of a chimney, which, when passing before the black soot, appears blueish, but as it ascends against the blue of the sky, it changes its appearance into a reddish brown. So the colour lake laid on blue will turn it to a violet colour; yellow upon blue turns to green; saffron upon white becomes yellow; white scumbled upon a dark ground appears blue, and is more or less beautiful, as the white and the ground are more or less pure.
Chap. CCXXXIV.—In what Part a Colour will appear in its greatest Beauty.
We are to consider here in what part any colour will shew itself in its most perfect purity; whether in the strongest light or deepest shadow, in the demi-tint, or in the reflex. It would be necessary to determine first, of what colour we mean to treat, because different colours differ materially in that re-
