Comparison of the experimentally observed and simulated evolution of the shocked column's density. Because these images are negatives, regions of high gas density are darker than regions of low gas density. The initial conditions of the column are shown at the far left; its initial diameter is about 6 millimeters. The shock wave moves from left to right. Time also proceeds from left to right, with 140 microseconds between frames. Time is measured from the moment the shock wave has left the downstream edge of the column. The entire experiment is over in less than a thousandth of a second. Good agreement is seen in the larger-scale structure of the density snapshots. Differences in the smaller-scale structure are the subject of continuing research. Note the pair of large vortices on the downstream side of the shock-wave/column interaction. The core of one of these vortices is used as a reference point for comparing the experimentally observed and simulated velocities late in the instability's evolution.