Why a bouncing ball rebounds at equal angles; the eye and mirrors
Ball rebound and spin, the dilating pupil, and rays through plain versus silvered glass.
The left and centre of the sheet study the mechanics of a ball striking the pavement: Leonardo argues that the angles of impact and rebound are equal, and reasons out why a struck ball takes a 'middle way' and also acquires spin as one part of it turns back while another goes on. On the right a small eye diagram carries his experiment on the black ring around the pupil, which he finds serves only to enlarge or shrink the pupil according to the light. At the lower left two ray diagrams distinguish a 'mirror without lead' (a ray passing through plain glass) from a 'mirror with lead' (a ray reflected from a silvered surface). The dense mirror-script surrounding the diagrams is fully transcribed in the notes.
On this page
Equal angles of impact and rebound
Leonardo opens by stating the principle he will demonstrate: that the angles of percussion and of rebound are equal. The heading governs the ball-and-pavement diagrams that follow below it.
The ball takes the middle way (d a - n c m - b)
A ball leaving d and rushing toward c would be driven on to b, but its flight is cut at c where it strikes the pavement m n. It cannot return the way it came, nor continue its begun flight, so it takes the middle way and leaps, converting the flight from c into a between equal angles.
Why the rebounding ball spins (S - o - n r - h c - f t m)
Leonardo asks why a struck ball does not always return along the way it came. He explains that as the part t n f strikes and turns back, the part t o n wants to go forward; the two being joined, one going and the other returning, a revolution or spin arises in the ball. A further passage adds that when percussion falls along m t the struck part becomes a pole, and the heavier and lighter parts, driven by equal power, contend with contrary motions that throw the ball upward.
The black ring around the pupil enlarges or shrinks it
By experiment Leonardo finds that the black or nearly black crinkled colour around the pupil serves no office but to increase or decrease the pupil's size: to grow when the eye looks into a dark place and to shrink when it looks at a light. He proposes holding a lamp near an eye and having it look now into darkness, now toward the light, to prove it.
Mirror without lead and mirror with lead
Two ray diagrams at the lower left contrast an unsilvered and a silvered glass. The first, a ray passing through a plate of glass, is labelled 'mirror without lead'; the second, a ray reflected from the surface, is labelled 'mirror with lead'.