Designed by Campbell and Obstfeld to take frame and multifocal measurements, the City Rule is an optical ruler that enables the user to accurately measure the following: 

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• Boxed Centre

• Boxed Lens Size

• Conventional Inset

• Geometrical Centre of Segment

• Geometrical Inset

• Monocular Centration Distance (This is not a term or measurement defined in a British Standard)

• Fitting Height

• Segment Bottom Position

• Segment Depth

• Segment Diameter

• Segment Height

• Segment Size

• Segment Top Position

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Supplied with instruction manual