  Quasi-elastic responses in nuclear matter and in $^{12}$C and $^{40}$Ca
nuclei are calculated in ring approximation to investigate the finite size
effects on the electromagnetic quasi-elastic responses. A method to simulate
these effects in infinite systems calculations is proposed. The sensitivity of
the results to the various terms of the residual interaction is studied. The
results of nuclear matter RPA calculations are compared with those obtained in
ring approximation to evidence the importance of the exchange terms.
