| "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 Ophthalmoscope (1864) [1] |
The Paris inch, or pouce, is an old unit of measure, that among other uses, was common for giving the measurement of lenses.[1] The Paris inch could be subdivided into 12 ligne (Paris line), and 12 Paris inches made a Paris foot.[1] The Paris inch and Paris foot could be abbreviated with " and ' like some other inch/foot units.[1] It was larger than the English inch and the Vienna inch, although the Vienna inch was subdivided with a decimal, not 12 lines. [1]
A Paris inch is equal to 27.069 millimetres.[2] The Paris inch was also known as a pouce.[3]
A famous example of its use is in the first Great Refractor. It had a lens made by Joseph von Fraunhofer which was 9 Paris inches in diameter, which works out to about 24 cm or 9.6 inches (English inches) aperture achromatic lens (the largest of its day).[4][5]