  This paper derives the macroscopic electric and magnetic fields and the
surface susceptibilities for a metasurface, starting from the microscopic
scatterer distribution. It is assumed that these scatterers behave as electric
and magnetic dipoles under the influence of the incident radiation.
Interestingly not only the retarded electromagnetic fields from oscillating
dipoles are relevant to pass from the microscopic to the macroscopic
representation, but the advanced fields must be considered too. It is found
that the macroscopic fields are the sum of the incident fields plus the
radiation-reaction fields acting on a single scatterer. Both the local fields
and the radiation-reaction fields are necessary to fix the electric and
magnetic surface susceptibilities.
