The translation used was crappy. It says in King James and substantially the same in Douay-Rheims-I just don't agree with your understanding of the parable. Not that I know of any other meaning for it. To me, it just seems too stretched. "The eye is the lamp of the body." Kind of strange translating that to: If you understand reality you'll see clearly.
"If thine eye be single." who translated that to healthy? And why would they translate the emphasized singleness of the eye into a plural 'eyes'?
The single eye is the spiritual eye, the inner intuition, the knowledge of truth. It can indeed be seen in the countenance of the person, at least by the more aware types, and it indeed gives an impression of giving light to the entire being, the whole body. Certainly the expression in the eyes would give it away, but it does not originate in the eyes or their health, but from the inner light of the realized person.
And of course, the corollary, "if thine eye be evil, thy whole body will be full of darkness" is also true. He is referring to the spiritual, inner man.
And, "how great is that darkness" is a lamentation of the condition of a person whose inner light is in fact dark.
