Filar micrometer by William Herschel, 1780-1800

Eye-piece micrometer inscribed E.6 in wood mount with brass frame divided circle (120 divisions numbered from 0 degree to 60 degrees) and steel screw, 2 threads in position

Dated 1780-1800, this eyepiece filar micrometer was made by the astronomer, William Herschel for the telescopes he used. In addition to building his own reflecting telescopes, Herschel made his own micrometers. This measuring instrument uses a set of fixed and moveable wires to determine angular distances in the eyepiece of the telescope. In operation, the calibrated handle was used to move the parallel wires delineating the object being measured. The distance was then calculated from the number turns of the handle that is attached to a fine threaded rod connected to the framework holding the moveable wires. Like Herschel's own eyepieces, the body of this instrument is made of cocoa, a tropical hardwood still used for woodwind instruments today.