Why is the 1931 standard observer called a 2 degree observer?

The 1931 standard observer data were derived from colour-matching experiments with an arrangement that meant that the stimuli activated an area of the retina of 2 degrees. The distribution of rods and cones is not uniform over the surface of the retina and this implies that the tristimulus values obtained from the 1931 data are strictly only valid for observations made under 2 degrees viewing conditions. This is equivalent to viewing a small coin held at arm-length and does not correspond particularly well with the viewing conditions often used in the coloration industry.