PRECISION SPHEREOMETER, French, mid-19th century. Standing 6-1/2″ tall, made of brass with a steel lead screw, the spherometer has three tapered legs, central pin on the fine thread precision screw, and “micrometer” readout with vertical scale graduated every half-millimeter, and circular scale (on a six-spoked ring) graduated every 0.001 millimeter (!) The central pin presses against an upper double lever arm with very high amplification factor, so that repeatable positive contact is reliably indicated. The instrument thus measures the central height, and therefore the sphericity, of a surface. Condition is fine noting nicks around the scale edge, and crazing to the original lacquer finish. The most sophisticated early mechanical spherometer we have seen.
Ask the Dealer
David and Yola Coffeen both have enjoyed academic careers, as planetary astronomer and as linguist/educator. But since 1982 (yes, 1982!) they have been full-time dealers in early scientific and medical instruments, under the name Tesseract. Selling primarily by catalogue (over 100 issued so far) they also have a web presence at www.etesseract.com, and can be contacted at firstname.lastname@example.org.