Several things must be carefully pointed out to medical specialists of the human eye In any discussion on this subject with them, otherwise opthalmologists, optometrists, etc., will mistake your description of unusual structures in the pigeon eye for ignorance and dismiss everything said by you in its entirety.

The flanged membrane, an adaptation for long distance focusing peculiar to Racing Pigeons, is an organ that covers a doughnut-shaped sphincter rruscle that is independent of the iris and is not present in the human eye. That unique sphincter muscle, in the eye of (all or some?) birds, is lodged beneath the inner edge of the iris and does not occur within it.

Whether the doughnut sphincter is found in other birds besides the pigeon we will leave to our readers to discover for themselves. We ordered several advanced books on bird anatomy from our local library but will probably not get them until after these pages go to press.

Our speculations, on considering the information above, now follows:
Since sphincter muscles in the eye permit the pigeon to control the pupil opening, well-developed sphincters, the ones built into the iris and also the big one beneath it, allow a bird to open and close the pupil readily. The highly elastic doughnut sphincter under the iris - the visible portion of that sphincter or its flanged membrane being the Circle of Adaptation - permit the pupil to close even smaller. Moreover, this large, well-developed sphincter beneath the iris, fitting snugly on top of the lens, prevents light from seeping in around the inner edges of the pupil and does this best with a non-reflecting, all-absorbing, black-in-color membrane. Plentiful black fibers, growing throughout the membrane, as the sphincter contracts and relaxes, allow for more effective control of the pupil.

The advantage the pupil has in closing to a pinhole is this it helps a bird flying over great distances focus on distant objects. A pinhole eliminates spherical aberration. A curved lens is primarily adapted for viewing objects close up. It's also the best lens for viewing objects away from the axis of sight on the periphery of vision. Curvature in a lens, however, is not effected when focusing on distant objects. Nor is curvature in the cornea desirable. Distant objects, from the standpoint of the structure of the eye, can be considered points at infinity. To bring distant objects into focus the eye must be able to modify the curvature of the lens along with that of the cornea, reducing both.

One way to do this is to flatten the cornea (the "twitch", which we will describe later) and decrease the size of the aperture so that light will pass through the lens over a small, relatively flat area. Light rays parallel to the line of sight striking a lens at points far from the axis of sight (central sight line that passes through the center of the pupil) do not pass through the principal focus for rays close to the axis of sight. This defect of a curved lens for viewing distant objects, in telescopes with refracting lenses, Is called spherical aberration.

Before the time of parabolic reflecting mirrors, which largely replaced refracting lenses, a diaphragm was needed to limit the portion of a curved lens to a small area surrounding the axis of sight - just as the flanged membrane protruding in the Pupil of the pigeon eye, the Circle of Adaptation, does for this bird.

Depending on atmospheric conditions and the height of flight, pigeons with good eyes may be able to focus on and identify landmarks as far away as 75 miles. Under ideal atmospheric conditions, when pigeons fly very high on being first i  released, they probably can see even farther than that. We remember as happened to us a few years back, while standing on Lookout Mountain in Chattanooga, of being told that the mountain we saw in the distance was over 100 miles away. That, of course, was looking from one peak to another. However, the eyes of a pigeon are even better than ours. We will say more on that later; but that pigeons can see far is supported by the fast speed of return during short races in good weather.

To keep all of what's been said in perspective, at this point we will say that even with excellent eyes a pigeon without a powerful body will never be able to take the punishment encountered when flying in a long race irregardless of how good its eyes. Why, then, do we place importance on the eyes? Because a pigeon with a strong body performs better when aided by good eyes. Such a pigeon has an edge and it does so not only because it can see farther.

Largely descended from the Eurasian Rock Dove (Columbia livia) the pigeon seems likely, despite Darwin's claim, to be an admixture of the Rock dove and several of its close relatives. Pigeons were probably first domesticated in Mesopotamia as far back as six thousand years. Neolithic man tamed animals as long ago as 8,000 B.C. For at least five thousand years (the earliest record of pigeon breeding can be traced to 3000 BC to the fifth dynasty of pharoahs in Egypt) some pigeons were used for carrying messages - probably first from battlefield to ziggurat in ancient Sumeria along the Tigris Euphrates - but flown over relatively short distances where vision played an important role in helping the pigeon to return home.

From the beginning those birds which saw their home loft easily were the ones to return soonest. They were also used for breeding. This selective process continued down through the ages to ancient Persia where the black Messenger Pigeon was reported to have originated. The traits for vision and homing ability, then, began to be associated early. Those birds able to see well and home well, the ones having in their brains a larger degree of magnetite, or other physical aspects that combine to form homing intelligence, the ability to hear low frequency sounds, the sense of smell, or whatever, were the ones to return home more often to be selected for breeding.

This continued thru the times of the Arabic empire, and their pigeon post, then during the last 150 years the selection process, with breeding for racing ability occurring in Europe to an unprecedented degree, became intensified. Twombly, who, in his book on eye-sign, reprinted and edited, for the most part, what F. Gidot had to say in 1911 on eye-sign, is largely the source of what follows.

Gidot, a Belgian, who had an intimate knowledge of what went on in the lofts of 19th century Belgium, said that almost all Racing Pigeons by 1890 were interbred with a Mr. Ulens' family of birds, which turned out to be one of the most Influential families In the formation of the modern racer.