Coddington magnifier
Encyclopedia
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 again minimizes spherical aberration
. This allows for greater magnification than a conventional magnifying glass, typically 10× up to 20×. Most single lens magnifiers are limited to 5× or so before significant distortion occurs. The drawback is that the diaphragm groove reduces the area seen through the magnifier.
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. In 1829, Henry Coddington
brought the Wollaston-Brewster lens into general notice, and further refined the design by modifying the shape of the groove, though Coddington laid no claim to being its inventor.
Magnifying glass
A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle ....
consisting of a single very thick lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
with a central deep groove diaphragm at the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, thus limiting the rays to those close to the axis, which again minimizes spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
. This allows for greater magnification than a conventional magnifying glass, typically 10× up to 20×. Most single lens magnifiers are limited to 5× or so before significant distortion occurs. The drawback is that the diaphragm groove reduces the area seen through the magnifier.
History
In 1812 William Hyde WollastonWilliam Hyde Wollaston
William Hyde Wollaston FRS was an English chemist and physicist who is famous for discovering two chemical elements and for developing a way to process platinum ore.-Biography:...
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
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
as the glass. He therefore used a sphere from a single piece of glass with a deep groove cut in it. In 1829, Henry Coddington
Henry Coddington
Henry Coddington was an English natural philosopher, fellow and tutor of Trinity College, Cambridge and Church of England clergyman....
brought the Wollaston-Brewster lens into general notice, and further refined the design by modifying the shape of the groove, though Coddington laid no claim to being its inventor.