Simple and compound derivative shadows; shadow made visible in the air
Pyramidal, columnar and dilating shadows, and how sunbeams through a chink reveal shadow in dusty air
This leaf gathers four short chapters on the geometry of shadow. It first notes that a pyramidal shadow from a parallel body narrows the further from the body the simple derivative shadow is cut, then classifies simple derivative shadows (595) as three straight-sided kinds: the finite pyramidal, the infinite columnar, and the infinite dilating, born respectively from an opaque body smaller than, equal to, or larger than the light. Chapter 596 divides compound derivative shadow into columnar and dilating, and chapter 597 explains that shadow becomes visible in misty or dusty air, as when sunlight enters a dark room through chinks and the shadow shows between the rays. A large decorative initial L sits in the upper text.
On this page
The three simple derivative shadows (595)
The simple derivative shadow is of three straight-sided kinds: one finite in length, which is pyramidal, and two infinite, one columnar and the other dilating. The converging (pyramidal) shadow arises from an opaque body smaller than the light, the columnar from one equal to the light, and the dilating from one larger than the light.
Shadow made visible in the air through a chink (597)
Shadow can be seen through misty or dusty air; this is shown when the sun penetrates through chinks into dark places, for then the shadow is seen interposed between the two or more sunbeams passing through those openings.
Compound derivative shadow (596) and the darker near shadow (598)
The compound derivative shadow is of two kinds, columnar and dilating. A further chapter begins the argument that the derivative shadow is darker the nearer it lies to its opaque body or to its primitive shadow, because its boundaries are more distinct at their birth.