Assumed evolution of detection efficiency toward the LMC with experiment lifetime T for ﹩T=2.1﹩, 6, 20, and 60 years (dashed lines). The efficiency is assumed to increase with T for longer Einstein diameter crossing times ﹩\hat{t}﹩ up to a maximum efficiency level of 0.5. The actual detection efficiency of Alcock et al. (1997a) after 2.1 years observation is shown by the solid line.