<P> For example, while the L cones have been referred to simply as red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish - yellow region of the spectrum . Similarly, the S - and M - cones do not directly correspond to blue and green, although they are often described as such . The RGB color model, therefore, is a convenient means for representing color, but is not directly based on the types of cones in the human eye . </P> <P> The peak response of human cone cells varies, even among individuals with so - called normal color vision; in some non-human species this polymorphic variation is even greater, and it may well be adaptive . </P> <P> Two complementary theories of color vision are the trichromatic theory and the opponent process theory . The trichromatic theory, or Young--Helmholtz theory, proposed in the 19th century by Thomas Young and Hermann von Helmholtz, as mentioned above, states that the retina's three types of cones are preferentially sensitive to blue, green, and red . Ewald Hering proposed the opponent process theory in 1872 . It states that the visual system interprets color in an antagonistic way: red vs. green, blue vs. yellow, black vs. white . Both theories are now accepted as valid, describing different stages in visual physiology, visualized in the diagram on the right . Green ← → Magenta and Blue ← → Yellow are scales with mutually exclusive boundaries . In the same way that there cannot exist a "slightly negative" positive number, a single eye cannot perceive a bluish - yellow or a reddish - green . (But such impossible colors can be perceived due to binocular rivalry .) </P> <Table> <Tr> <Th> Cone type </Th> <Th> Name </Th> <Th> Range </Th> <Th> Peak wavelength </Th> </Tr> <Tr> <Td> </Td> <Td> β </Td> <Td> 400--500 nm </Td> <Td> 420--440 nm </Td> </Tr> <Tr> <Td> </Td> <Td> γ </Td> <Td> 450--630 nm </Td> <Td> 534--555 nm </Td> </Tr> <Tr> <Td> </Td> <Td> ρ </Td> <Td> 500--700 nm </Td> <Td> 564--580 nm </Td> </Tr> </Table>

Who proposed that the eye must have receptors for three colors of light