Among equal lightenings of light, such proportion will hold from obscurity to darkness of the generated shadows as will hold from obscurity to darkness of the colours where such shadows join.

Of the motion of the shadow.

a - b c - d e

The motion of the shadow is always faster than the motion of the body that generates it, the luminous body being immobile. It is proved: let the luminous body be a, the shadowing body b, and the shadow d. I say that in equal time the shadowing body b moves to c while the shadow d moves to e; and the proportion of velocity to velocity made in the same time is as the length of motion to length of motion. Therefore, by the proportion that the length of motion made by the shadowing body b as far as c has to the length of motion made by the shadow d to e, such do the said velocities of the motions have among themselves.

luminous - shadowing - shadow

But if the luminous body is equal in velocity to the motion of the shadowing body, then the shadow and the shadowing body will be of equal motions among themselves.

And if the luminous body is faster than the shadowing body, then the motion of the shadow will be slower than the motion of the shadowing body.

But if the luminous body is slower than the shadowing body, then the shadow will be faster than the shadowing body.