Color Vision
Theories of Color Vision

The Four Color Theory

In 1872 the English physiologist Ewald Hering put forth his own theory of color vision known at the �Four Color Theory.� Hering proposed that color vision operated on an opponent bases. He stated that there were three basic kinds of receptors on the retina, red-green, yellow-blue, and black-white, each containing a different biochemical visual substance. When light entered the eye, the result was that in each type of receptor the quantity of visual substance would begin either to increase (assimilate) or decrease (dissimilate) depending on the spectral nature of the light. For example, with respect to the red-green receptors, he assumed that long wavelength and short wavelength light caused dissimilation and produced the sensation of red, while middle wavelength light caused assimilation and produced the sensation of green. If the incoming light contained as much intensity in the long and short wavelengths as in the middle wavelengths, assimilation would equal dissimilation, and the receptors would in effect be neutralized. Likewise, for the yellow-blue receptors the long wavelength light that comprises two thirds of the spectrum caused assimilation and produced the sensation of yellow, while the short wavelength portion of the caused dissimilation and produced the sensation of blue. White light would produce equal amounts of assimilation and dissimilation so that no hue would be perceived.

The Ladd-Franklin Theory

This theory, stated in 1932, was a compromise between the three and four-color theories and was an evolutionary approach to color vision. This theory states that, in the beginning, there was sensitivity to light alone without discrimination of colors.

As evolution proceeded, a substance sensitive to light became subdivided, forming two subsidiary substances�one of which was sensitive to long waves, and the other to the short waves in the visible spectrum. The long waves gave a yellow sensation, whereas the short waves gave a blue sensation. Mixed light affected both substances, producing white.

A second step in evolution divided the yellow substance in the same way into two parts, giving different sensations for red and green and a combined sensation for yellow.