Paris inch

"The Paris inch is commonly employed as the unit by which to express the focal length of lenses, and it cannot, therefore, be dispensed with" The Paris inch or pouce is an archaic unit of length that, among other uses, was common for giving the measurement of lenses.

[1] The abbreviations are the same as for other inch and foot units, i.e.: for Paris foot a single prime symbol ( ′ ), for Paris inch a double prime symbol ( ″ ) and for Paris line a triple prime symbol ( ‴ ),[1] The Paris inch is longer than the English inch and the Vienna inch, although the Vienna inch was subdivided with a decimal, not 12 lines.

[1][2] A famous measurement made using the Paris inch is the lens measurement of the first great refractor telescope, the Dorpat Great Refractor, also known as the Fraunhofer 9-inch.

The 9-Paris inch diameter lens was made by Joseph von Fraunhofer, which works out to about 24.4 centimetres (9.59 English inches).

[3][4] The term for telescopes persisted even in the 20th century, with a telescope listed in the 1909 Sears Roebuck catalog of having 25 lignes diameter aperture, or about 56 mm (5.6 cm).