Small light gives sharp shadows; the speed of a cast shadow
A uniform medium assumed; twin shadow-pyramids from two lights; a slow object throws a swift shadow.
Leonardo prefaces that in all his propositions the medium between the bodies is understood to be uniform. One figure shows that the smaller the light, the more distinct the boundary between lit and shadowed regions; another treats an opaque body between two lights, giving the proportion of the two shadow-pyramids to the diameters and distances of the lights. A final labelled diagram states that when an object moves slowly before a light, its cast shadow moves swiftly, in proportion to the distances object-to-light and object-to-shadow.
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A uniform medium is assumed throughout
In all the propositions to follow, the medium found between the bodies is understood to be uniform in itself. This holds the transmitting air constant so that only the geometry of light and body varies.
Smaller light gives a sharper shadow boundary
The smaller the luminous body, the more distinct is the meeting of the luminous convergence with the shadowy one. A near-point light produces a crisply bounded shade behind the obstacle.
Twin shadow-pyramids from two light sources
When two opposite shadow-pyramids from one and the same body are double in darkness and similar in figure, the two lights that cause them are of double diameter and stand at double distance from the opaque body. The middle figure sets a small opaque body between two lights whose rays cross.
A slow object casts a swift shadow
If an object moves slowly before the light and its cast shadow falls far away, the motion of the derivative shadow bears the same proportion to the motion of the object as the space between object and light bears to that between object and shadow. Thus a slowly moving object throws a swiftly moving shadow.