WATSON 'SCREW MICROMETER EYEPIECE':

c. 1915

Maker: W. WATSON & SONS LTD.

DESCRIPTION:

This micrometer is made of lacquered brass with a silvered drum calibrated to 1/100 of a mm. Although the engravings indicate a double speed control, this example has only one. The instrument has the 23 mm (0.9173 inch) eyepiece diameter but as shown in the image to the left can be used with an adapter for the larger diameter tubes as in the the Grand Van Heurck microscope in this collection. Unlike the engravings, this example has the improvement of a clamp to secure the device in a fixed position on the optical tube. Inside there is a 'comb' scale with a stationary line to the left and another line which moves right and left, controlled by the micrometer screw.

HISTORY:

This type of micrometer was called a Ramdsen Eyepiece micrometer after the inventer, Jesse Ramsden. The original model moved a spider web along a scale of 'combs.'   This was then replaced by a thin wire, and then, as in this model, a fine line scribed on glass. Dallinger suggested a fixed line as well as a moving line, this modification being found here. Modern versions are often referred to as 'Filar' micrometers and now have the fine line moving past an engraved numbered scale instead of a 'comb.'   An example of the latter instrument made by the Swiss company Wild is in this collection, as are earlier versions of micrometers.

A 'screw micrometer eyepiece' was offered in Watson Catalogs from no later than 1893 through at least the 1930's. The design likely changed somewhat, but the same engraving was used continuously. In the early twentieth century, it was available both in Aluminum and Brass.