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Filar Micrometer


Micrometer with cover, 6 lens attachments and 3 small lens parts in wooden box with padding. Micrometer back has threaded circular opening to fit over a telescope of diameter 8.5 cm. Inside the opening is a smaller aperture leading to external lens opening. Across the opening is two very fine threads can be observed (one across the center of the external lens opening and one to the side – about 5mm apart). Threads have the appearance of spider silk. Micrometer front has smaller lens opening for lens attachments (with a cover). The opening is part of a movable piece adjusted by parallel bar with small knobs on each end. On each end of the micrometer itself are two large circular knobs (for micrometer readings), marked from 0 to 100, each connected to a small circular dial (for millimeter readings) on the front of the instrument, marked from 0 to 50. When turned the knobs move the fine threads on the inside of the instrument.

Accession Number: 2012.ast.7

Alternative Name:

Astronomical Micrometer, Bi-Filar Micrometer, Double Filar

Primary Materials:

Micrometer: brass, other metal (alloy), fine thread – possibly spider silk. Attachments: brass and glass. Box: wood and felt.


Engraved on micrometer: “T. Cooke & Sons, York. England.” Label on inside of box reads: “Micrometer Screw. 1 turn = 23.944”

Dimensions (cm):

Height = 5.5 x Width = 25 x Length = 9.8


Measure the angular distance between distant objects. Used to measure the distance between double stars.


Good. Slight fraying on internal threads.

Associated Instruments:

Manufacturer: T. Cooke & Sons

Date of Manufacture: late 1800s


University of Toronto Department of Astronomy and Astrophysics

Additional Information and References:

Historical Notes: