Perspective: the axis of the angle of incidence on a cylinder
Four equal angles about the point of incidence, and when the eye fails to see the reflection
Continuing the study of reflection, Leonardo asserts that the ray of a luminous body makes its angle of incidence among four equal angles, which define the axis of that angle. On a columnar (cylindrical) object the axis falls among four right angles, each equal to its counterpart, with d a f equal to c a b and b a e to b a g. He also notes a case where, because angle a c m is not equal to its opposite m c b, the eye o does not see the point n at c. Several small diagrams of cylinders struck by rays, labelled with letters, illustrate the propositions.
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The luminous ray and the four equal angles of incidence
The ray of the luminous body makes the angle of incidence among four equal angles, that is, it defines the axis of that angle. The proposition is headed 'Perspective'.
Axis of incidence falling among four equal angles
c a d is the angle of incidence upon the columnar object e g, and the axis b a of that angle terminates at a among four equal angles, each equal to its corresponding one: d a f is equal to c a b, and b a e to b a g.
When unequal angles keep the eye from seeing the reflection
Here the angle a c m is not equal to its opposite m c b, so the eye o does not see n at c. The axis of the angle of incidence nonetheless always falls among four right angles.