THE POWELL & LEALAND No 1

ACCESSORY: Ramsden-type Filar Micrometer Eyepiece

Authors: Jurriaan de Groot & Barry Sobel

Editor: Joseph Zeligs

This fine micrometer eyepiece is made to the design originated by Jesse Ramsden. As the silvered dial is turned, one fine wire moves across a group of points towards another fine wire fixed in the center of the field. The ocular can be focused on the lines and points(all shown to the right) by pushing it in or pulling it out of its barrel. The instrument fits the inner tube diameter of the P & L No 1 tube without an adapter. This micrometer itself has been adapted to this purpose, as the outer larger diameter tube is held in place by the original smaller tube of 29mm in diameter. This micrometer is unsigned, but virtually identical to the design of Powell & Lealand's. Ramsden's filar micrometer, initially used for telescopes, was first described in 1779:
The Description of Two New Micrometers. By Mr. Ramsden, Optician; Communicated by Joseph Banks, Esq. P. R. S. Author(s): Mr. Ramsden and Joseph Banks Source: Philosophical Transactions of the Royal Society of London, Vol. 69 (1779), pp. 419-431