but it’s likely that they will tend towards one becoming concave and the other convex. The problem of making a truly flat surface in this way was solved in the 1830s by Joseph Whitworth ...

A bed that is level in our terms presents a flat surface that is parallel to the ... but for some other materials it could be convex or concave or even have ripples all over the place.

Our main focus is on the production of spherical lenses with concave, flat or convex surfaces, prisms and prismatic lenses of the highest quality materials, optical filters using Schott materials, ...