English inch, Paris inch calipers

Thanks, Peter!

Here is an unusual set of calipers used to measure the focal length of lenses, the unit of which is the Paris inch. English inches are also included for your convenience (or confusion).

Wikipedia. "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)

Overview of various methods of measuring focal length- here.