The shifting shadow of a globe and the perspective of colours
Why a cast shadow varies infinitely in shape and size, and the painter's rule for colour and linear perspective.
A page from Leonardo's Treatise on Painting continuing a demonstration that a cast shadow changes in both shape and size as its light source moves: here the shadow of a globe labelled d shifts from d-f to e as the light passes from b to a. Because the site of the shadow is a continuous quantity divisible to infinity, its shape and size vary infinitely. A small pen diagram at the upper right shows the globe and its lettered rays. Two painter's precepts follow, on matching the diminution of colour, figure and linear perspective, and on darkness defined as the total privation of light.
On this page
A globe's cast shadow shifts from d-f to e
As the light moves from b to a, the shadow of the globe d, which reached from d to f, shifts to e, so it changes in both size and shape because it now falls on a differently shaped place. Since the site of the shadow is a continuous quantity divisible to infinity, the shadow's shape and quantity are variable to infinity.
Match the diminution of colour and linear perspective
The painter is told not to diminish the perspective of colours more than that of the figures in which they arise, nor the linear perspective more than that of the colours, but to follow both according to the rules of the eighth and the seventh. In nature the perspective of colours never breaks its law, whereas the perspective of magnitudes is free, so a small hill may stand near the eye and a great mountain far off.
Darkness as the total privation of light
The obscurity of darkness is the total privation of light; and between light and darkness, being a continuous quantity, the gradation is variable to infinity. Between darkness and light, the text adds, there is a pyramidal power.