Wherret et al.¹ have derived a general condition for the existence of intrinsic optical bistability, stipulating that the absorption length of the sample, αL, must be less than 0.18. Nonetheless several research groups have observed optical bistability in samples with absorption lengths significantly above this limit.² It has been suggested² that the observed bistability is the result of an initial self-focusing of the incident beam brought about by thermal refractive index change which then induces a bistable effect in the final portion of the sample. If the focal length of the laser induced thermal lens is comparable with the sample thickness, then the reduction of the laser beam size could then induce bistability at a lower threshold in the final portion of the sample. We have carried a systematic test of this hypothesis, using a modified version of the Z-scan technique, on a commercially available semiconductor-doped glass filter.