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That portion of a body of uniform breadth which is against a lighter
background will look narrower [than the rest].
[
4] e is a given object, itself dark and of uniform breadth; a b
and c d are two backgrounds one darker than the other; b c is a
bright background, as it might be a spot lighted by the sun through
an aperture in a dark room. Then I say that the object e g will
appear larger at e f than at g h; because e f has a darker
background than g h; and again at f g it will look narrower from
being seen by the eye o, on the light background b c. [Footnote
1
2: The diagram to which the text, lines 1-11, refers, is placed in
the original between lines 3 and 4, and is given on Pl. XLI, No. 3.
Lines 12 to 14 are explained by the lower of the two diagrams on Pl.
XLI, No. 4. In the original these are placed after line 14.] That
part of a luminous body, of equal breadth and brilliancy throughout,
will look largest which is seen against the darkest background; and
the luminous body will seem on fire.
2
45.
WHY BODIES IN LIGHT AND SHADE HAVE THEIR OUTLINES ALTERED BY THE
COLOUR AND BRIGHTNESS OF THE OBJECTS SERVING AS A BACKGROUND
TO
THEM.
If you look at a body of which the illuminated portion lies and ends
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