Spherometers are used to measure the radius of curvature of the surface of a lens. If the radii of both surfaces of the lens and the index of refraction of the glass are known, the focal length can be found using the Lens-Maker's Equation.

   When the three outer legs are placed on a flat surface and the central leg screwed down to touch the surface, the zero reading is obtained. The vertical scale reads in millimeters, and the scale around the edge of the circular disk may be divided into as many as 1000 graduations. The design dates from 1810 and is due to Nicolas Fortin and made for him by the French optician Robert-Aglaé Cauchoix.
  The spherometer at the right is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University, and was purchased from Duboscq of Paris, probably about 1875.

   At the left is a beautifully-finished spherometer sold by James W. Queen of Philadelphia. Simpler models were sold for about $28; this may be the $50 model listed (but not illustrated) in the 1881 and used "to measure exceedingly small thicknesses of solid films." This apparatus, fitting into a box with a sliding lid, is in the Greenslade Collection.

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