The Lewis effect — normally observed as the characteristic overshoot on the leading edge of narrow resonance yield curves from thick targets — is a consequence of the discrete nature of the energy loss process for the particles penetrating the target. We present an expression for the size of the intrinsic Lewis effect, in which the energy and medium dependence are included through the stopping power and straggling constant. We discuss how various experimental parameters, such as contamination layers on the target surface, the beam energy spread, the “half-atom” effect, and nuclear resonance width transform the way in which the Lewis effect expresses itself in thick target excitation curves.