The following figure is from Pirenne (VISION AND THE EYE, Chapman and Hall, Ltd, 1967 edition). Shown is an abstracted section from Figure 2.4 (p.30) “Horizontal section through a region of a fixed human retina containing the fovea” from Osterberg. The figure shows the cones of the fovea and the appearance of the initial rods that are being statistically introduced beginning at the foveal edge. The reference notes the appearance of the “first rod” at ~ 0.13 mm from the foveal center. Just beyond this point, one observes the first, statistically still rare, rod/rod apposition. I would propose following the nano-antenna explanation of this work that this represents a short wavelength light interaction site that has been misinterpreted as a “blue sensitive cone” (please remember that the use of “color” should not be used for light interaction at this point on the retina – a narrow band of light wavelengths interact that have not yet been synthesized into the hues of color).
This statistical and rare distribution of, what now can be seen as, short wavelength light detection sites on the retina seems in consonance with historical findings of their scarcity, location, and distribution.
I add a Figure from George Wald (with legend) that he used to illustrate the magnitude of the change in the distribution of cones and rods in the region near the foveal edge. This is a tricky place to make measurements!
It will be pointed contesting this conclusion that the “gene has been identified for the blue cone”. I would answer that there is certainly truth in this oft used type of statement but that it is incomplete in saying nothing about the mechanism or mechanisms involved in effecting the action of this genetic information. Genes code for, in cooperation with many other genes, the synthesis of protein structures. I would argue that if one talks about “blue sensitive cones” the identified gene would involve protein/structural mechanisms involved in formation of the diameter of the rod receptor. It is this diameter setting the receptor appositional nano-antenna distance that determines short wavelength (improperly termed “blue”) response ( I would add that any change in this rod dimension would alter the cone/rod ratio that would in-turn move the location of the geoemtrically-determined mid band point on the retinal surface – leading to ‘color blindness’?). If one talks about the gene that codes for the “blue pigment”, I would propose that the mechanism involved would be alteration of the physical size of the protein structure (or ”cage”) for the retinal molecule formed by the rhodopsin moeity. All is geometry!