I wasn't sure my sphere was simple glass and from what I see on the web it could be anywhere from refractive index 1.5 to 1.7. I did a reasonably measured scene to just match it visually (the lighting doesn't matter just the ray bending) and got:
Index is around 1.55. I don't think more resolution is likely with this method
Nick P suggested I use a laser pointer and that would have been easier and more accurate. Wish I read that first :) I'll try it on the glass sheet.