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form more than of any other shape; and when
lighted, whether by the sun or flame, or by
apertures admitting light, must have two relative
extremes of light and shadow, two balancing tints,
the illuminated and did reflected, divided by 3.
middle tint or the aerial. The effect of illumin-
ation by flame or aperture, differs from that of
the sun in this respect; the su11 illuminates with
parallel rays, which fall over all parts of the
enlightened side of the subject, While the light
of a flame or an aperture only strikes directly
on the nearest pointof the object, producing an
effect which more or less resembles the illumin-
ation of the sun in proportion to the distance
and dimensions of the object.
" Let us then suppose a ball to be the object
on which the light falls, in a direction of forty-.
fwe degrees or the diagonal of a square, and at
a -right angle from the ball to the place where
you stand. One half of the ball will appear
illuminated, and the other dark. This state of
the two hemispheres constitutes the two masses
of light and shadow. In the centre of the mass
of light falls the focus of the illumination in the