Who said there was something wrong with it? Seems fine to me. What information does this provide?

That a ray of light, that happens to please Parsifal by bending according to the wiki-function, would never curve back into the sky.

Cool. Can you explain how you discerned that? Would the graph be more parabolic if it would bend back?

I don't understand you now. Again, I can have everything wrong, but this curve in the link isn't it, as a ray of light would look like from the side? For any choosen value of the constants it would have exactly one inflection point. If we assume now that y = the vertical distance from the earth plane - as I allways did -, you can see with your eyes, that it doesn't curve up again.

But there's another thing: rays emitted from the sun in different angles relative to the y-coordinate must show a different bending behaviour. So Bollybill and RealScientist did right in saying the Bishop constant cannot be a constant. It must itself be a function of the angle.