**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).