“…Similar considerations apply in the tangential case, for which it can easily be shown that D(s)=A f f exp {ik(W(x+s/2, y)-W(x-s/2, y))} dx dy (12) In both of these cases, then, the integration only needs to be performed over onehalf of the area, so that the new area of integration is as shown in figure 5 (a), for the sagittal case, and figure 5 (b) for the tangential case . …”