  We present a novel physical interpretation of the level sets of the
(canonical) lapse function in static isolated general relativistic space-times.
Our interpretation uses a notion of 'constrained test particles'. It leads to a
definition of gravitational force on test particles and to a previously unknown
uniqueness result for the lapse function.
  In Section 5, we discuss 'photon spheres' in static isolated relativistic
space-times and relate them to the level sets of the lapse function.
