Sphere Micrometer Adapter

This is an adapter for a micrometer head (I used a Starrett V263MRL, since I had it to hand).

It permits the measurement of the radius of a sphere, and caps the micrometer face to avoid marring the lens I was measuring. It provides three points on a circle for convex curves, and four(of which you use three) on a concave curve. Stick these into a bit of python, such as [this one](https://www.geeksforgeeks.org/equation-of-circle-when-three-points-on-the-circle-are-given/) and it'll spit out the radius, which makes modelling lenses much easier. I've included the resulting model so you can see what I'm up to.

##Preparation:
1. Print, clean, and assemble
1. On a flat surface, measure where the micrometer thinks the tip touches the flat. Record that as your zero point.
1. Accurately measure the inside and outside diameters of the bell.
1. Accurately measure the outside diameter of the tip.

##Measuring a convex surface:
Place the bell anywhere on the surface, and measure th