A derivative shadow ringed by lit ground is darker than the primitive
Reflected light from the illuminated field brightens the primitive shadow but not the enclosed derivative
The leaf finishes the congregable/disgregable distinction, noting the congregable always makes an impact smaller than the primitive shadow while its disgregable part does the reverse. Chapter 602 proves that a derivative shadow wholly or partly surrounded by an illuminated field is darker than its primitive shadow: with light a, opaque object b c, and wall d e receiving the derivative shadow at n m, the lit strips d n and m e reflect light back into the primitive shadow b c, so the enclosed derivative n m, seeing no light, stays dark while the primitive is brightened. Chapter 603 begins to argue that a primitive shadow not lying on a flat surface is not of even darkness. A pen diagram carries the letters a, b, c, d, e, m and n.
On this page
The enclosed derivative shadow is darker than the primitive (602)
Let the light be a, the opaque object b c, and the wall d e which receives the derivative shadow at the part n m; its remainder d n and m e stays lit by a and reflects light into the primitive shadow b c. Not seeing light a, the derivative n m remains dark, while the primitive is illuminated by the surrounding lit field, so the derivative is darker than the primitive.
Congregable impact is smaller than the primitive shadow
Closing the previous chapter, Leonardo states that the congregable always has the shadow's impact smaller than the primitive shadow, while its disgregable part does the contrary.
Uneven darkness of a shadow off a flat surface (603)
A new chapter proposes that a primitive shadow not joined to a flat surface will not be of equal darkness, setting up the proof with the primitive shadow on object b c d, its derivative shadow f g, and its illuminated field e f g h.