Grunow Spherometer
From the collection of National Museum of American History. Free to download, adapt and use — no permission needed.
A spherometer is a device used primarily for measuring the curvature of objects such as lenses and curved mirrors. They were produced from the 18th to the early 20th centuries, for use by astronomers and opticians and are still mentioned on websites today. The spherometer is a micrometer comprised of three legs that define a plane. A central point can be raised or lowered to just touch the surface. The distance this central point is above or below the flat plane made by the three legs is read off the vertical scale on the device. This distance is then translated into a number that describes the radius of curvature of the surface. Spherometers can also be used to measure the thickness of a flat plate or the amount a flat surface deviates from being truly flat. (See the introduction to the spherometers for the details of the uses and mathematics of this device.)
National Museum of American History’s own description
