What effect does convergence
alone have in the perception of
distance and depth? The best
method of isolating this cue
is to hold accommodation
constant by using a mirror
stereoscope and then to vary
the degree of convergence. As
the illustration at left shows,
the researcher can vary the
angle of convergence merely
by changing the lateral position
of the cards on which the
"stereograms" appear. (In this 
case, the "stereograms" are 
identical pictures of a single 
object or figure presented to 
each eye, since we are not 
interested in retinal disparity.) 
Because only the angle—–not 
the distance—–at which the 
cards are viewed changes, the 
lenses remain the same shape—–
that is, accommodation 
remains unchanged. If the 
figure is luminous and is 
viewed in the dark, we would 
then seem to have isolated 
convergence as the only 
varying factor.