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.

[Footnote: The diagram belonging to this chapter is No. 2 on Plate
III. In the original it is placed between lines 3 and 4, and in the
reproduction these are shown in part. The semi circle above is
marked _orizonte_ (horizon). The number 6 at the left hand side,
outside the facsimile, is in the place of a figure which has become
indistinct in the original.]

On the intensity of shadows as dependent on the distance from the
light (150-152).