A pocket comparator is an optical device for measuring and inspection comprising a

loupe A loupe ( ) is a simple, small magnification device used to see small details more closely. They generally have higher magnification than a magnifying glass, and are designed to be held or worn close to the eye. A loupe does not have an attached h ...

and a

reticle A reticle, or reticule also known as a graticule, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the screen of an oscilloscope, ...

. The instrument was developed and manufactured by the

Bell & Howell Bell and Howell LLC is a U.S.-based services organization and former manufacturer of cameras, lenses, and motion picture machinery, founded in 1907 by two projectionists, and originally headquartered in Wheeling, Illinois. The company is now ...

Company, but similar instruments of other names are made by other manufacturers. PocketComparator2

It is used for: * Linear measurements in fractions of an inch * Circular measurements in fractions of an inch * Radius measurements * Angle measurements * Narrow line width measurements * Circular measurements in decimals of an inch * Linear measurements in inches * Linear measurements in millimeters Measurements are performed by bringing the surface of the reticle as close as possible to the work inspected.The Pocket Comparator: A New Measuring Instrument, in: INDUSTRIAL QUALITY CONTROL VOL.6 NO.5, March 1950 pp. 22-23

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