Acuity for apparentvernieroffset

Observers viewed an apparently moving vertical bar, displayed sequentially at a series of discrete positions on an oscilloscope face. When the plotting sequence was such that the upper half of the bar was always displayed slightly before the lower, the bar appeared to be broken and offset at the middle, with the upper segment leading the lower (although they are actually displayed at identical horizontal positions). Acuity for detecting this illusionary offset is very fine indeed, almost as fine as that for detecting real offsets. It is further shown that such an offset can be detected only if the bars are seen to be in motion.