Attached Shadow vs. Separate Shadow; Rays from a Point
Rays diverge from a single point, so a body between light and wall casts an enlarged shadow; attached shadow never leaves the body
A text page from the fifth part of the Treatise on Painting, carrying a small geometric diagram of light rays diverging from a point. It explains that a body close to the light, also lit by the luminous air, has blurred shadow edges, and that because rays issue from a single point and spread as they travel, a body between light and wall is always thrown into an enlarged shadow. It then distinguishes attached shadow, which never leaves an illuminated body, from separate (cast) shadow, illustrated by a ball a braccio from a wall.
On this page
Bright air blurs the edges of a large shadow
The thing that casts a large shadow is near the light and is lit both by the light and by the luminous air around it; this air leaves the shadow's bounds confused. Ambient illumination softens the edges of a near-cast shadow.
Rays issue from one point and widen the cast shadow
Experience confirms that luminous rays come from a single point and disperse in a circular course through the air, widening as they recede. A body placed between the light and the wall is therefore carried into a larger shadow, because the rays touching it are wider by the time they reach the wall.
Attached shadow versus separate shadow
Attached shadow never departs from an illuminated body: a ball in the light always keeps one part of itself in shadow, which no change of position removes. Separate shadow may or may not be cast; set the ball a braccio from a wall with the light opposite, and it throws upon the wall as much shadow as covers the ball's near side.
Diagram of rays diverging from a point
A small geometric figure at the centre of the page shows straight rays fanning out from a point on the left toward a vertical line at the right, illustrating how light spreading from a single point widens as it travels. It accompanies the argument about the enlargement of cast shadow.