NavList:

Fred you wrote:

"The basic conclusion both with Frank's searching and on that other list
is the same: the angular resolution of the eye is finer than the
spacing of the rods & cones.  Frank's resolution limits also were in
accord with those on the other list."

 

The standard imaging resolution of the eye is just what you would expect from diffraction limits and the spacing of the cones. Vernier acuity, which goes waay beyond normal resolution, apparently involves only certain very specific discrimination tasks. Do these have direct relevance to sextant use? Some should, but it depends on how specific these tasks are. For example, it is clear that the eye can distinguish discontinuities in straight lines at very high angular resolution. So if I were to use a distant power line (or other narrow straight feature) to get the index correction of my sextant, I should be able to get very accurate results. But what about the sea horizon? Can the eye detect discontinuities in the visual horizon (which is not a line but instead a boundary between light and dark) with the same hyperacuity?

And:
"In neither of them, was the question of how the eye achieves this
resolution addressed in much depth."

 

It does not appear that there is any solid theory for the origin of hyperacuity (vernier acuity). One rough model apparently suggests that the vision system is integrating over many cones to accomplish this trick.