Quasi-elastic responses in nuclear matter and in C-12 and Ca-40 nuclei are
calculated with the ring approximation to investigate the finite-size effec
ts on the electromagnetic quasi-elastic responses. A method to simulate the
se effects in infinite-systems calculations is proposed. The sensitivity of
the results to the various terms of the residual interaction is studied. T
he results of nuclear matter random-phase approximation calculations are co
mpared with those obtained in the ring approximation to demonstrate the imp
ortance of the exchange terms.