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straight line from its cause to the object or place it falls upon.
FIRST BOOK ON LIGHT AND SHADE.
On the nature of light (130. 131).
1
30.
The reason by which we know that a light radiates from a single
centre is this: We plainly see that a large light is often much
broader than some small object which nevertheless--and although the
rays [of the large light] are much more than twice the extent [of
the small body]--always has its shadow cast on the nearest surface
very visibly. Let c f be a broad light and n be the object in
front of it, casting a shadow on the plane, and let a b be the
plane. It is clear that it is not the broad light that will cast the
shadow n on the plane, but that the light has within it a centre
is shown by this experiment. The shadow falls on the plane as is
shown at m o t r.
[Footnote 13: In the original MS. no explanatory text is placed
after this title-line; but a space is left for it and the text
beginning at line 15 comes next.] Why, to two [eyes] or in front of
two eyes do 3 objects appear as two?
Why, when you estimate the direction of an object with two sights
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