When buying Convex Glass (Bubble Glass) frames, I generally ask, "What is the depth of the glass?" Most sellers state, "I don't know how to measure that". I've been disappointed several times when a "bubble glass" frame is really just (almost) flat glass.

It's actually quite simple to measure. Here's an overview on the initial set-up.

1. Just turn the glass over on a flat surface (bubble down)
2. Lay a straight edge (ruler #1) across the edges of the glass.
3. Using another ruler (#2), measure from the center of the glass up to the straight edge.

The photo below shows a close-up of the distance considered "the depth (curve) of the glass"

Read the dimension where ruler #2 is touching rule #1

Thanks for looking and GOOD LUCK in your eBaying.