Using equations (1), (2) and (5),
verify equations (3) and (4).
Taking the unit square geometry of
Figure 3(a),
assume emission points at the four centerpoints of the surfaces and normal
emission.
Thus, for the left-hand, vertical surface, take: 
and
.
According to equations (3) and (4),
attempt to compute the intersection points for surfaces 2--4, etc.
