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The light which falls on a shaded body at the acutest angle receives
the highest light, and the darkest portion is that which receives it
at an obtuse angle and both the light and the shadow form pyramids.
The angle c receives the highest grade of light because it is
directly in front of the window a b and the whole horizon of the
sky m x. The angle a differs but little from c because the
angles which divide it are not so unequal as those below, and only
that portion of the horizon is intercepted which lies between y
and x. Although it gains as much on the other side its line is
nevertheless not very strong because one angle is smaller than its
fellow. The angles e i will have less light because they do not
see much of the light m s and the light v x and their angles are
very unequal. Yhe angle k and the angle f are each placed
between very unequal angles and therefore have but little light,
because at k it has only the light p t, and at f only t q;
o g is the lowest grade of light because this part has no light at
all from the sky; and thence come the lines which will reconstruct a
pyramid that is the counterpart of the pyramid c; and this pyramid
l is in the first grade of shadow; for this too is placed between
equal angles directly opposite to each other on either side of a
straight line which passes through the centre of the body and goes
to the centre of the light. The several luminous images cast within
the frame of the window at the points a and b make a light which
surrounds the derived shadow cast by the solid body at the points 4
and 6. The shaded images increase from o g and end at 7 and 8.
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