To us, the answer is supposed 
to be obvious because we 
respond appropriately to the 
depth cues, which in this case 
would be interposition, 
perspective, and familiar size. 
In the actual figure, however, 
simple line drawings were used 
to represent the objects. 
Texture was absent. In fact, all 
of the cues are, at best, weak in 
the original figure. Even for us 
the figure is ambiguous. Thus it 
is hardly surprising that the 
subjects in the experiment 
often said that the spear was 
aimed at the elephant.

    One should also be cautious 
about using size or shape 
perception as an indicator of 
depth perception in pictures. 
That is because there is only a 
tendency toward constancy 
(rather than perfect constancy) 
in viewing pictures, even 
pictures rich in effective depth 
cues (to say nothing of pictures, 
such as Hudson’s, in which the 
cues are so poor that the 
elephant looks as small as its 
relatively small visual angle 
would lead one to predict). 
Thus, if members of some 
African tribes report that an 
animal in the picture near the 
horizon looks much smaller 
than the one of the same 
species in the foreground of the 
picture, it should not be 
assumed that they are 
perceiving any differently than 
we do.