Those interested in this concept of vision should perhaps read George Wald’s “On the Blue-Blindedness in the Normal Fovea“ (J. Opt. Soc.Am, Vol.57, No.11, November 1967). Revisiting it in the course of writing the past day’s comments I find that the retina that Wald describes corresponds to a large extent with my description and a great deal of his writing can be read in this context. I had previously noted that the retina described in Wald’s Nobel Lecture (I believe it was his Figure 15) appears very much as the geometric retina that I define.
But first, two remarks. (1.) In the context of identifying the color detection centers and morphology of the retina, Wald maddeningly says nothing of the, what has to be related, optical physics of the image formation process used by the eye. It seems just assumed (again, I guess!) that the retina forms the image plane of the “camera” that is the eye. After describing a retina much as I see it, namely, that color detection centers are spatially separated on the retinal surface in areas (rings) around the fovea, Wald just leaves it at that not relating this morphology to any concept as to how an image is formed! (2.) Wald in his discussion uses the only light detection model available to him at the time, namely, that the trichromicity of the vision process must be equated to “three types of color sensitive cones”. He does, however, introduce the term “blue receptor function” (p.1294, italics mine) in the place of “blue sensitive cone”.
At the risk of being accused of taking Wald’s thoughts out of context I will transcribe some sections of his text. I truly believe that these sections mirror and summarize the thrust of the paper.
p.1289..from the Abstract:
“The blue cone system falls in sensitivity from the border of the photopic zone – the functionally all-cone areas – to a minimum, usually to extinction, at it’s center”…”Also the red – and green cone systems display the opposite gradient; their sensitivities decline regularly from the center toward the borders of the fovea and beyond”
This describes the morphology that I define with even more geometric precision..This is undoubtedly the basis for the retinal sensitivity curve that Wald presented in his Nobel lecture. It is truly beyond me how anyone seeing this – color sensitivity segregated into specific areas or “rings” on the retinal surface did not think to relate this diffractive surface to the wavelength refractive properties of the eye!
p.1290:..I have been anticipated!
Wald, (after calling it a misconception), quotes: “Wilmer had thought to explain color vision in terms of only two kinds of cone, or cones, and cone-like rods, the third color mechanism, that for blue, involving the cooperation of ordinary rods (ref).. Hence, he too expected to find no blue receptors throughout the entire photopic area…”
Wald defines the “photopic area” as the region of the retina “dominated by cone vision” and generally subtending an angle of 1-1.5 degrees. He goes on to say “that the photopic area of the retina contains large numbers of such blue-sensitive cones”. I believe, in the context of my comment yesterday, that he is mistaken in ascribing this function to cones – it is in this critical area of the retina that rods are beginning to intrude on the cone matrix and…the logic of my thoughts yesterday.
(ref): E.N.Wilmer, Nature 153, 774 (1944), and E.N. Wilmer, J. Theoret. Biol., 1, 141 (1962))
p.1294:
“It seems therefore that the absence or near-absence of blue receptor function is characteristic only of the center of the fovea. It is a matter, not primarily of size of field, but of foveal topography”
p.1296:
“The general gradient of cone concentration in the fovea therefore runs counter to the gradient of blue-cone sensitivity. At the center of the fovea, where con3es are most dense, blue-cone sensitivity is minimal, and blue-cones are usually entirely absent. Conversely, at the borders of the photopic area, where the blue-cone sensitivity is highest, the total number of cones has decreased markedly. This opposed distribution is reflected in a converse pattern of relationships that involve the red and green cones”
p.1298:
“…whereas the sensitivities of the red and green-mechanisms are highest at the center of the fovea, and decline regularly with distance from it, the blue cones show just the opposite gradient of sensitivity, rising from a minimum at the center of the fovea to one half degree to one degree out…”
p.1298:
“Congenital blue-blindedness is such a rarity that its existence was doubted….the most optimistic estimate of the incidence of congenital blue-blindedness is 1 in 13,000…”
On p.1299 Wald recognized the problem of chromatic aberration in the eye:
“Any lens made of one material exhibits chromatic aberration; it refracts short wavelength light more strongly than long wavelength light, and hence brings blue light to a shorter focus than red. Color-corrected lenses can of course be made by using two glasses differing in refractive index; but, so far as is known, no animal has yet succeeded in developing a color-corrected lens. Though the cheapest of cameras have color-corrected lenses, the lens system of the human eye – as Newton observed – has no color correction whatever”
p.1300:
What we take, therefore, to be normal, trichromic vision is the particular property of an annular zone of the central retina, lying between the blue-blind fixation area and a red-green blind zone some 20-30 degrees out. The special importance of this central zone is that we depend upon it almost exclusively for all phase of photopic vision, and that, all testing of color vison is confined within it, rarely extending mor ethan 2 degrees beyond the fixation point”
GCH
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