1. The variation of threshold with field area was measured in fields homogeneous in rod-cone composition. At 15 degrees above the fovea, an increase in field diameter from 1 degrees to 5 degrees reduces the threshold sevenfold, at 25 degrees above the fovea tenfold. 2. These changes are shown to follow qualitatively from simple statistical properties of the retinal mosaic. Analytic treatment leads to the expression, (A - n(t))(k) I = C, in which A = area, n(t) = constant threshold number of elements, I = threshold intensity, and k and C are constants. This equation describes the available data accurately, and is the general form of previous empirical area-threshold formulae.