Every shadow cast by a body has a central line directed to a single
point produced by the intersection of luminous lines in the middle
of the opening and thickness of the window. The proposition stated
above, is plainly seen by experiment. Thus if you draw a place with
a window looking northwards, and let this be _s f_, you will see a
line starting from the horizon to the east, which, touching the 2
angles of the window _o f_, reaches _d_; and from the horizon on the
west another line, touching the other 2 angles _r s_, and ending at
_c_; and their intersection falls exactly in the middle of the
opening and thickness of the window. Again, you can still better
confirm this proof by placing two sticks, as shown at _g h_; and you
will see the line drawn from the centre of the shadow directed to
the centre _m_ and prolonged to the horizon _n f_.

[Footnote: _B_ here stands for _cerchio del' orizonte tramontano_ on
the original diagram (the circle of the horizon towards the North);
_A_ for _levante_ (East) and _C_ for _ponete_ (West).]