A center, in the dynamic sense of the term, acts as a focus from
which energy radiates into the environment. The forces issuing
from the center are distributed around it in what we shall call
the center’s visual field. In the simplest case the energy is
evenly distributed in all directions; the field of forces is
centrically symmetrical. Constraints can narrow the field to
particular directions, as in the case of a floodlight . . .