2

CONSTANCY
A camera obscura: a small 
opening in the side yields an 
inverted, two dimensional 
picture of the three 
dimensional scene outside.  To 
see pictures in a camera 
obscura clearly, the entire 
room must be dark.

In the opening years of the 
seventeenth century, the 
French philosopher René 
Descartes described an 
experiment that would reveal 
the visual image as it appears 
on the retina of the eye. One 
could remove the eye of an ox 
and place it in a frame, 
scraping the back of the eye to 
make it translucent, he said. By 
looking at the retina, one could 
see how the world was projected 
on it. To understand perceptual 
constancy--our tendency to see 
the properties of objects as 
unvarying despite ever-
changing retinal stimuli--and 
most other aspects of 
perception, we first must know 
how scenes actually are 
represented on the retina.