Coddington magnifier

A Coddington magnifier is a magnifying glass consisting of a single very thick lens with a central deep groove diaphragm at the equator, thus limiting the rays to those close to the axis, which minimizes spherical aberration.

Most single lens magnifiers are limited to 5× or so before significant distortion occurs.

In 1812 William Hyde Wollaston introduced a much improved version of the earliest magnifiers consisting of a spherical glass by employing two hemispheres of glass mounted together with a small stop between them.

Sir David Brewster found that Wollaston's form worked best when the two lenses were hemispheres and the central space was filled up with a transparent cement having the same refractive index as the glass.

He therefore used a sphere from a single piece of glass with a deep groove cut in it.