Crossing Bodies Appear Multiplied; Birds' Wings
Two opaque bodies in intersecting motion seem to be three, with X and V motion diagrams
Leonardo opens with an aphorism urging the reader to shun any study whose product dies with its maker. He then describes a visual effect: two opaque bodies moved against each other in intersecting motion appear as three, while two moving without intersecting motion appear as four, as happens with the wings of flying birds. Two small diagrams at the foot of the page label crossed lines (X) as "intersected motion" and angled lines (V) as "simple motion."
On this page
Aphorism on lasting work
The note begins with a maxim: flee that study whose resulting work dies together with the one who made it. It sets a moralizing frame before the observation that follows.
Apparent multiplication of bodies in crossing motion
Two opaque bodies moved one against the other in intersecting motion appear as three; one thing may appear as two, and two moving without intersecting motion appear as four. Leonardo treats the effect as a phenomenon of vision produced by overlapping moving forms.
Applied to the wings of flying birds
Leonardo closes the observation by comparing the effect to the wings of flying birds, whose beating produces the same apparent multiplication of moving forms. It links the visual rule to the motion of a bird in flight.
Intersected motion (X) versus simple motion (V)
A pair of crossed lines forming an X is labelled "intersected motion," while two lines angled into a V are labelled "simple motion." The diagrams distinguish the two kinds of movement invoked in the note above.