Nonius horopter measurements were made on three normal subjects using both physically displaced (real viewing) and stereogrammetrically displaced (projected viewing) test objects. Three subjects with ocular anomalies were tested using the projected viewing arrangement with and without 5% overall magnifier before the right eye, and seven complete measurements of the projected viewing horopter were made on a fourth subject with an ocular anomaly over the period of 1 month. These latter individual plots showed large irregularities that changed location from day to day. Plotting the mean and standard error of the means of these seven sets of data produced a smoothing out of the horopter locus. Plotting each individual horopter setting in the sequence of measurement and monitoring eye position indicated that all of the irregularities and regional horopter changes found are most likely introduced by changes in fixation disparity (vergence eye movements).