Understanding Rods and Cones

• Duplicity theory begins with an understanding of the components of the eye. The retina is made up of two kinds of cells that are sensitive to light: rods and cones. These cells are named for their relative shapes. Cones and rods each serve a different purpose for vision and duplicity theory seeks to explain these differences.
  
  

Description of Duplicity Theory

• Proponents of duplicity theory believe that the cones, while not highly sensitive to light, discriminate colors and allow the eye to distinguish minor and distant details. Rods, on the other hand, are extremely sensitive to light, but cannot distinguish colors or process fine details. Because cones and rods have different functions, scientists, including Isaac Newton and Max Schultze, determined that there must be two kinds of vision, daytime and nighttime vision. Cones are primarily responsible for daytime vision since they can easily discriminate colors and minute details. Rods are responsible for nighttime vision since they can process images, albeit not distinctly, in very low light.
  
  

Evolution of Duplicity Theory

• The origin of the duplicity theory dates back to Isaac Newton's research in the 17th century. Newton began to examine differences in the way the eye responds to light and dark. Max Schultze, however, is considered the father of the duplicity theory. In 1866, he became the first scientist to distinguish rods from cones and extrapolated that there is a "duplicity" of vision since the rods and cones behaved differently.
  
  

Explanations of Colors

• Schultze's work explained that rods and cones not only process light differently, they also process colors differently. Cones, which are better for acute vision, can process all colors, while rods can process only dark colors, primarily purple. Subsequent research tweaks Schultze's theory. R. Granit conducted a study in 1947 indicating that rods process colors that cones do not. Therefore, if rods process purple (and other dark colors), cones do not. The rods and cones serve mutually exclusive functions, according to Granit.
  
  

Counterargument

• Though the duplicity theory has many supporters, it is not universally accepted as fact. Some research---most notably Frederick William Edridge-Green's 1920 study of vision---indicates that the duplicity theory may be flawed because the rods and the cones may simply process different colors, regardless of the amount of light present. Therefore, what previous researchers interpreted as daytime versus nighttime vision may actually be light-colors vision versus dark-colors vision. That is, the rods of the eye may process dark images, such as an eggplant, even in daytime.