A Coddington magnifier is a
magnifying glass
A magnifying glass is a convex lens—usually mounted in a frame with a handle—that is used to produce a magnified image of an object. A magnifying glass can also be used to focus light, such as to concentrate the Sun's radiation to create ...
consisting of a single very thick
lens
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
with a central deep groove diaphragm at the ''
equator
The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
'', thus limiting the rays to those close to the axis, which minimizes
spherical aberration
In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical ...
. This allows for greater
magnification
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...
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 Wollaston
William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable i ...
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
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
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.
''History of Science''
Williams, Book 4, chapter V.
See also
* Stanhope lens
References
{{Reflist
External links
''Build a Coddington''
Magnifiers
Lenses