On common perspective: apparent thickness and background colour
How uniform or varied ground colours make an object appear of varied thickness
Under the heading 'On common perspective', Leonardo sets out rules for how the apparent thickness of an object depends on the colour of its background. An object of uniform thickness and colour seen against a ground of non-uniform colour will look of non-uniform thickness, and one of varied colour against a uniform ground will likewise look of varied thickness. The greater the variety of colours in the object or its ground, the more its thickness seems to vary, even when the objects are in fact of equal thickness.
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Background colour alters apparent thickness
A body of uniform thickness and colour, seen against a ground of non-uniform colour, appears of non-uniform thickness. Where the colours of the object or the ground differ most, the apparent variation in thickness is greatest, even for bodies of truly equal thickness.
A rule of common perspective
The note is titled 'On common perspective' and gives a painter's rule about judging widths against their setting. A thing of varied colour on a uniform ground will look of varied thickness just as a uniform thing on a varied ground does.