The Neural Control of Vision
C. The Retina

In the fovea, a small central region in the retina, that affords high-acuity vision, there are only cones with an intercone distance of just 2.4 micra that comes to 0.7 minutes of arc. There are 200,000 cones per square millimeter in this region. Just 5 degrees out there is a 10-fold drop in the number of cones per unit area and the intercone distance increases to 7.5 micra (2.2 min of arc). This occurs because the cones here have a larger diameter and because they are further apart to give room for the rods that now appear interspersed between the cones.

The rods, that are for night vision, have their own rod bipolar cells and connect with ganglion cells via a special set of amacrine cells. As a result of this arrangement, at locations where the rods and cones are intermingled in the retina, ganglion cells get input from both rods and cones. Thus in the daytime these ganglion cells are driven by the cones and at night they are driven by the rods.