When observers have only 
the cue of convergence to go 
on, their reports about distance 
are quite inaccurate and 
inconsistent. If the 
convergence movement of their 
eyes is inward, which ought to 
create the impression that the 
figure is approaching, subjects 
are just as likely to say the 
figure is receding or is not 
changing distance at all as they 
are to say that it is 
approaching. However, reported 
changes in the figure’s size 
vary quite predictably; with 
increased convergence, for 
example, the figure will always 
be seen to diminish in size.

    These findings about size 
and distance may seem 
counterintuitive, but there is a 
logic to each of them. The 
findings for size are precisely 
what we should predict on the 
basis of Emmert’s law, that the 
perceived size of an object of 
constant visual angle varies 
directly with perceived 
distance. If we assume for the 
moment that convergence is a 
cue to distance and the eyes 
converge more sharply, the 
object viewed will be 
interpreted as coming nearer. 
Since the visual angle is 
constant in this case (because 
the figure remains at one 
distance), and since distance is 
perceived to decrease, then, 
according to Emmert’s law, 
perceived size must decrease. 
The opposite result obtains 
when the eyes diverge.