What it takes for the eye to function is a complex process using several proteins and energy transfers. Below is a simplified outline of how vision works.

When light first strikes the retina, it causes a molecule called 11-cis-retinal, to change shape. This molecule forces a change in rhodosprin, a protein of which molecule is part. This changed  protein (metarhodopsin II) combines with another protein to form a larger, more complex protein called phosphodiesterase. This complex protein accquires the ability to slice a specific molecule called cGMP.

Now in the back of the eye, there is a membrane that also binds cGMP. This membrane regulates the passage of sodium ions (Na+).

As the the complex proteins increase, the level of cGMP also decreases. As the level of cGMP decreases, the membrane in the back of the eye senses the loss and closes the ion gate, lowering the amount of sodium ions in the eye. This causes an imbalance in the electric charge across the eye, forcing a pulse of ions down the optic nerve to the brain, which the brain interprets as vision.
 
This is a fine and dandy way to see however, there has to be a way to shut the process down or else all the cGMP and sodium ions would be quickly depleted. Here is how the eye stops the process.

First, the ion membrane also lets in calcium ions (Ca+) with the sodium. When the sodium ion concentration drops off, so does the calcium ion concentration. Phosphodiestrease acts more slowly in a low calcium enviroment.

Second, a protein called guanylate cyclase starts producing cGMP when the calcium ion concentration falls.

Thirdly, the metarhodopsin II protein is modified by an enzyme so that it cannot continue to bind to the next protein in the chain.

By these methods, the eye carefully regulates just  how much of the chemicals are used up in the vision process. Now how do you ask does the eye return to the original state of the 11-cis-retinal? Afterall, that molecule starts the whole thing off and unless the eye replinshes the supply, then vision would quickly be used up.

A molecule  called trans-retinal first is severed from the protein and two hydrogen atoms are added by an enzyme,  forming trans-retinol. Then a second enzyme converts the molecule to 11-cis-retinol. Finally, a third enzyme removes the two hydrogen atoms to reset the whole system to the beginning with 11-cis-retinal.

Scientists of the 19th century knew the physical structure of the eye very well but not the chemical complexity that would come only in the middle 20th century. However, even back then, scientists knew enough that the eye must have all its parts or face partial to total blindness.

Darwin addressed the evolution of the eye in his original work. He makes the assumption that the eye is a fairly simple lens-type structure. The evolution of which could easily be seen in the eyes of "lower" life forms, for there are eyes which only can detect a difference in light and dark. Then others bend over on themselves and so it can start detecting the direction of the light source. This progression leads upward until then there is the human-type eye, which is a camera-type shutter lens. It all seemed perfectly logical to the scientists and laymen of Darwin's day because they could not even imagine the complexity of how the eye actually works.

Now imagine if cGMP was not present in the eye. Or perhaps if rhodopsin was not there. Maybe 11-cis-retinal was absent. How would the eye function? The answer is that it wouldn't because the complex chain of vision would be broken. Yet this is how Darwin proposed things happened.....step by step. The first protein reacted with something else which mutated to form another beneficial protein, which mutated to form another beneficial protein, which mutated.......and so on and so on.....
But the eye is an irreducible system, meaning that all the parts of its function must be present or else the whole system fails. One single protein or even two or three proteins in the process of vision, cannot claim to be the root of vision. Without the whole system as it currently is, vision would fail.

Therefore by Darwin's own criteria, the eye fails his test of a complex organ which cannot be shown to have developed step by step.
 

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