Although the dark-room 
experiment seems to be a pure 
test of the accommodation cue 
alone, it is not. There is a 
strong link between 
accommodation and 
convergence (the two eyes 
working together) that even 
this experiment cannot 
eliminate. When one eye 
changes accommodation from 
near to far or from far to near, 
the other eye, when occluded, 
will change its direction 
appropriately so that both eyes 
are converged on the same 
point in the scene. (If you hold 
a finger lightly over the closed 
eye as you change your fixation 
from a near to a far object, or 
vice versa, you can even feel 
the closed eye turn.) To test the 
efficacy of both oculomotor 
cues together, observers can be 
asked to view a single object in 
a dark room with both eyes. 
Under these conditions, when 
all the information about 
distance seems to derive only 
from convergence and 
accommodation, observers can 
judge distance with a 
reasonable degree of accuracy 
up to about 10 feet. That the 
oculomotor cues, operating 
together, are indeed effective is 
borne out by the fact that 
constancy of size is maintained 
within this distance under the 
same conditions.