Figure 2. Response versus intensity curves for frog rods and Carassius cones. Black circles: rods; red and green circles: cones. Green solid curves through filled circles mark the most- and least-sensitive green cones. Red curves through empty circles show the range of scatter of red cone data. Curves were drawn by the least-squares method using Equation (10) for cones, and (10a) for rods. Graphs are plotted versus effective light intensity I_e as defined in Equation (6).