The Discussion Place

The retina has been put together backwards!

Unlike the retinas of octopuses and squids, in which the light-gathering cells are aimed forward, toward the source of incoming light, the photoreceptor cells (the so called rods and cones) of the human retina are aimed backward, away from the light source. Worse yet, the nerve fibers which must carry signals from the retina to the brain must pass in front of the receptor cells, partially impeding the penetration of light to the receptors. Only a blasphemer would attribute such a situation to divine design!" - Frank Zindler It's not my intention to offer rebuttal or to start a controversy or anything of the sort; this is strictly between you and me.  I've heard this argument before, which either goes, "God is so stupid he doesn't even know how to design an eye," or, "God must not have designed the eye, else he wouldn't have made such a silly mistake."

Regardless of your religious persuasion (which is none of my business), I'm sure you can see the fallacy in the argument, which is the arrogant assumption that one knows how to design an eye better than the one we all have.

To that assertion, let me just add the following observations:

1) The human eye is sensitive enough to detect, when dark-adapted, a single
photon.

2) The resolution of the human eye exceeds the theoretical diffraction limit (thanks to brain-powered image enhancement).

The design may be "inexcusable," but it sure seems to work well, don't you agree? During the last flight of the Mercury series, astronaut Gordon Cooper must have had exceptional atmospheric seeing conditions, or exceptional eyes, or both.  He claimed to have seen, from orbit, railroad tracks, wakes of ships,
highways, and even trails between huts of mountain villages. I was one of several scientists who whipped out our sliderules and determined that the things Cooper claimed he saw were beyond the diffraction limit, even under the most optimistic assumptions.   The responses tended to clump themselves into about three groups:

1) Those who declared, pompously, that Cooper couldn't possibly have seen what he claimed to see

2) Those who didn't think he could, but weren't about to tell Gordon Cooper he was dreaming it all

3) Those who thought maybe he somehow did manage to see them.

I was in the third group.  As it turns out, when you examine the theoretical background behind the diffraction limit, it's based upon the ability to resolve two _POINT_ _SOURCES_ of light.  It says nothing at all about seeing a reflecting straight line, like the wake of a ship or railroad tracks.

Further, the diffraction limit assumes that the peak of one diffraction pattern is at the trough of the other.  This is not really necessary.  In computerized image enhancement, we can now routinely separate point sources that are closer than the diffraction limit, by examining the shape of the
light-density curve.  There's no reason to assume Cooper's brain couldn't do the same thing.
Finally, they did some checking after the fact to see if ships were really where Cooper said they were; if railroad tracks and trails were where he said.  They were.
Re the design of the human retina, there's a very good reason for it that those who think they could do better or missing.

As it happens, I have an intimate exposure to it, since I suffered a detached retina just about the time I got out of college with my Ph.D., ca 1968.  The human retina has no blood vessels in it; it depends for its nourishment, strictly on an intimate
contact with the blood vessels lining the sclera of the eye. 

The problem with a detached retina is that the retina loses contact with the sclera, which means that the nerve cells will die, within a day or two for the fovea, within a week or two for the cells in the peripheral vision (less density <==> less need for energy).

I guess I should count myself fortunate that Frank Zindler, or others that use his argument, didn't design _MY_ eyes, else I'd be blind by now. 

It seems that whoever or whatever designed the human retina, got it right after all. He knew that making sure all those cells were well-supplied with oxygen and energy was more important than perfect optics.

And since, apparently, the optics are already plenty good enough anyhow, it was the right design
choice. Just so's you know.
One other little tidbit:  You may or may not remember that I like to raise baby birds, chickens, and ducks.  My ducks have eyeballs much, much smaller than those of humans (which means that the diffraction limit should be
proportionally worse). 

Yet they can not only see us humans at 200 yards, they can distinguish one human face from another.

How the heck do they _DO_ that?   In fact, how is it that a duck is more adept at distinguishing human faces than a _HUMAN_ is?  I have trouble understanding the evolutionary basis for that.

- Jack Crenshaw