The science of light and shadow: free and constrained light
Free vs. constrained light; primitive and derivative shadow; theory before practice
Both folios belong to Leonardo's science of shadow. He distinguishes 'free' light, which illuminates bodies openly, from 'constrained' light entering through an aperture or window, and works out how the length and direction of a cast ('separated') shadow depend on the height and size of the light relative to the shaded body. On 171r he sets out the paired terms primitive and derivative light and shadow, noting that a derivative shadow carries to the wall the form of its cause, and reminds himself to treat first the theory and then the practice, dense bodies before transparent ones. The sheet is crowded with small diagrams of light sources casting pear-shaped shadows, some lettered a and b.
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Free light and constrained light distinguished
There are two different lights: the free, which freely illuminates bodies, and the constrained, which lights bodies through some aperture or window. Every shadow must regard with its middle the middle of its light, a principle that governs the whole analysis that follows.
Length of a cast shadow depends on the light's height
A separated shadow will be longer when its light is lower, and shorter when its light is higher. The middle of the length of each shadow always directs itself to the middle of the luminous body, so a shadow separated from a body keeps the same nature and quality as the shadow joined to it.
Small light gives sharp shadows, large light soft ones
A light smaller than the shadowed body makes shadows terminated upon that body with little mixed shadow, seeing less than half of it. A light greater than the shadowed body sees more than half of it and makes much mixed shadow. When constrained light illuminates bodies equidistant from the window's center, the one set more transverse casts the greater shadow behind it.
Primitive and derivative light and shadow
Light is of two kinds, separated (illuminating the body) and joined (the lit part of the body), called primitive and derivative. So too shadow: primitive shadow is attached to the body, derivative shadow separates from it and carries to the walls the form of its cause. Every light and shadow draws itself into pyramidal lines.
Theory before practice; dense bodies before transparent
You must describe the theory and then the practice, and describe first the shadows and lights of dense bodies and then of transparent bodies. Every surface is the boundary of a dark body and multiplies upon itself the species of the things set against it, sending bright ones within bright.
A shadow's middle aligns with its light through the body's center
It is necessary that the middle of each shadow regard the middle of its light by a straight line passing through the center of the body. In the lettered figure the middle of the light is a and of the shadow is b, and shadowed bodies circumscribed by shadow and light have the middle of each directed to the body's center along one straight line.