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Preliminary results of continuing experimental study of factors affecting precision of centering black circular measuring marks in sharp, high-contrast targets with homogeneous backgrounds, subtending visual angles up to 45 min of arc, in photopic vision; maximum information content for centering task investigated was contained in ribbons approximately 1 min of arc wide around light areas of target and measuring mark; most precise pointings were made by selecting measuring mark to give minimum annulus width within these ribbons irrespective of target size; prototype polar-coordinate monocomparator using laser interferometer is described.