Distinct shadow edges; direct versus oblique impact of a shadow
Why remoter light sharpens a shadow's boundaries, and the converging, diverging and constant kinds of impact
The page continues the analysis of where a derivative shadow strikes a surface (its percussione, or impact). It states that a shadow's boundaries are most distinct at their birth (599), and are sharpest when the opaque body is more distant from the light. Chapter 600 divides the impact into direct and oblique, the direct always smaller and the oblique able to extend towards infinity; chapter 601 classifies the impact three ways relative to the primitive shadow as congregable (converging), disgregable (diverging) and observed (constant). Several small pen diagrams of tapering shadow-forms illustrate the direct/oblique and three-fold distinctions.
On this page
Sharpest shadow edges come from the remoter light (599)
A derivative shadow shows the boundaries of its impact most distinctly when its opaque body is more distant from the luminous body; its edges are clearer at their birth than in parts remote from that birth.
Direct and oblique impact of the shadow (600)
The impact of the derivative shadow has two varieties, direct and oblique. The direct is always smaller in quantity than the oblique, and the oblique can extend towards infinity. A marginal letter e labels this passage.
Congregable, disgregable and observed impact (601)
The impact of the shadow varies three ways relative to the primitive shadow: the disgregable (diverging) makes its impact greater than the primitive shadow; the observed (constant) keeps it equal; and the congregable (converging) yields two kinds of impact, one converging and one diverging.