Three experiments, using two sets of Nonius lines placed in a random-d
ot stereogram, indicated that Nonius alignment does not always reflect
binocular eye position and, thus, a caveat is necessary when Nonius a
lignment is used to monitor binocular eye position. We found that: (a)
two Nonius lines with visual line values that differed by up to 7.6 m
in of are can appear aligned; (b) the two lines of each of the two Non
ius sets continued to appear aligned despite a change in vergence angl
e of 5.9 min of are; and (c) the Nonius alignment reflected eye positi
on better, when the binocular dots near the Nonius lines were eliminat
ed. (C) 1998 Elsevier Science Ltd. All rights reserved.