What version of Poser are you using there? It looks like the back-side refraction is not reversed like it is in PP 2014 latest SR.

The result is you're dispersing even through the large face. 

Mine only shows the dispersion on the edge where the ray goes a long way through the glass. I didn't do anything to make that happen. When a ray goes across the front-back a short distance, the deviation is reversed quickly and no real bending happens. That's why mine only seems to bend light into colors along the edge.

So - Poser version matters here. PP2012 and earlier don't do the right math.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)