Measuring refractive index of a sphere

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.