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I prefer using refraction on the cornea rather than transparency, because when viewed from the side, a transparent surface does not "lift" the iris into view with transparency.

Well, in reality the cornea obviously is transparent, so it seems logical to make it transparent.
But I know what you mean. I think. You also have reflection to deal with too, not to mention fresnel.

The rest of what you said just went over my head. I model and draw, I don't program. ;-)