Found original part?

[kresty]

The back focal length is about 117mm, and the radius is 65mm. I think we can calculate the index of refraction from that, but I cant find the equation.

Oh, I missed that the focal length had been measured already.  That's what I was joking about.  

There are equations that tie focal length and index of refraction and lens shape together.  The refractive index can vary, maybe 1.49 to 1.6.  So, the material has a bit of an impact.  And a plastic FDM lens wouldn't be the same as the glass, and that wouldn't be the same as a resin lens.

I'm just being silly though, of course, since it doesn't really matter for our replica building   We don't need a perfect replica.  And even if you wanted to build a copy of the slide viewer, I'm pretty sure that the difference would be negligible.  Just change the adjustment of the focus a tiny bit.

I found a demo thingy for a lens that's convex on both sides: Molecular Expressions: Physics of Light and Color - Radius and Refractive Index Effects on Lens Action: Interactive Java Tutorial (fsu.edu) - It's not really a calculator though, but a formula are mentioned.