  In the trial \cite{BNL} of measuring the proton electric moment, storage
rings with electrostatic lattice have been considered. Here an overview is
given about the main parameters regarding such a kind of focusing. Beyond
confirming all the issues regarding this subject, a non-null element $M_{31}$
is introduced in all the $3\times 3$ matrices which deal with the vector
$(x,x',\Delta p/p)$ and its role is discussed.
