HAND HELD SMALL CODINGTON MAGNIFIER
YEAR: c. 1888
UNSIGNED
ORIGIN: English probably by Beck
DESCRIPTION:
The instrument is about 2 1/4 inches (57 mm) long. It is made of
'German Silver' also known as Nickel-silver. The optical element
measures about 1/2 inch (12.5 mm) in diameter, but the optical opening
is only about 3/8 inches (9 mm)in diameter. The handle has a small ring
at the end so it can be worn on a chain or lanyard around the neck;
this is not trivial as this instrument would be easily broken or bent
in an unprotected pocket. The lens is covered by a pair of covers which
automatically twist into position as the small disk is moved from the
end of the handle towards the lens. This is accomplished by both a
twisting motion and also a movement which moves the lens covers closer
to the lenses as the disk is moved.
This tiny instrument was shown in the Beck catalogs from 1888 to the
early 20th century. It was offered in brass, German silver, solid
silver, and even Gold.
HISTORY OF CODINGTON MAGNIFIERS
Until and even after the invention of the achromatic aplanatic microscope, small
pocket-sized magnifiers were in great demand. A simple uncorrected lens worked
well enough for very low powers, but once magnification exceeded just a few X,
distortions became apparent. These include spherical aberration, chromatic
aberration, and curvature of the field of view. Several different methods were
used to minimize these distortions, the most major one being spherical
aberration. The simplest and easiest, was simply to 'stop-down' the aperture
with a diaphragm. This was common in compound microscopes until the widespread
use of achromatic aplanatic objectives. In 1812 William Hyde Wollaston
introduced a much improved version of the earliest magnifiers
employing two hemispheres of glass mounted together with a small stop between
them. Sir David Brewster improved the design by using a single piece of glass
and cutting deep groove in it. In 1829, Henry Coddington popularized the
Wollaston-Brewster lens, and further refined the design by modifying the shape
of the groove, though Coddington never claimed to be its inventor. It was much
less expensive than creating a complex lens from different elements and then
cementing them together. The Coddington lens was sold in various sizes and
types from its inception, right into the 21st century; it is still available
today and is an inexpensive alternative to more complex magnifiers. It allows a
magnification of up to 20 diameters, whereas an ordinary magnifier is limited to
less than 5. Its major drawback is the reduced size of the field of view.
Despite the latter drawback it is far superior to the simpler Stanhope type of
magnifier. Improvements above this design generally are multi-element
magnifiers which increase the field of view and flatten the image across that
field. Such improved magnifiers are often Steinheil or Hastings triplets. These magnifiers produce a higher quality image than the Coddingtons, but are also much more expensive to make.