An Exercise In Retinal Organization

I had thought (and I believe written) that a retina composed of an array of receptors of a single size would respond only to a single wavelength, i.e., have a monchromatic response. In engineering terms such a retina would represent a “highly tuned, high Q” antenna array. This much is certainly true. Let’s imagine that the physical diameter (all that matters) of the receptors of such a retina to be of the same as human retinal cones, i.e., ~1 micron. The monchromatic response would then be at approximately 700 nanometers. If we were to substitute this retina for the admixture of two sizes of receptors (cones and rods) on the evolved human retina, all would still be well at the central fovea (zero degrees of retinal angle). The eye could still process the “Marr outline sketch” of the perceived scene. But, as one proceeded outward from the fovea, things would not be well at all. Such a retina would not be “tuned” to- and would not respond to, the wavelengths refracted by the body of the eye to larger retinal angles – the 550 nanometer wavelength at ~7 degrees, for example. Moreover, there could be no “Land color signal” derived from brightnesses (proportional to the density of “receptor antennas”) – and therefore color (probably even red) would not be perceived. This teaches that the sensation of color follows from a retina composed of two diameters of receptors and the specification of a geometrically-defined mid-band point.

One should see from this exercise how the refraction of wavelengths within the eye is intimately related to the evolution of the color sensing human retina formed of two diameters of receptors.