Robert Fantz made an important discovery in 1963 that advanced the ability of researchers to investigate infants' visual perception: Infants look at different things for different lengths of time.  

Fantz placed infants in a "looking chamber," which had two visual displays on the ceiling above the infant's head.  

An experimenter viewed the infant's eyes by looking through a peephole.  

If the infant was fixating on one of the displays, the experimenter could see the display's reflection in the infant's eyes.

This allowed the experimenter to determine how long the infant looked at each display.