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small bone-handled Coddington magnifier small bone-handled Coddington magnifier



This small magnifier is made of bone and brass. It is unsigned and bears no hallmark. There are two lines scribed around the lens housing and the Coddington lens is held in the brass lens housing by a knurled cap screwed to each end. It is about 91 mm in length. The aperture of the lens is about 11 or 12 mm in diameter. Unlike the Stanhope magnifier, this lens is symmetrical and can be used from either side.


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 aberation. 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.