object glass micrometer for Cassegrain telescope
Date: 1758-1764
Inventory Number: 0059
Classification: Object glass micrometer
Subject:
Maker: Dollond & Son (1752 - 1761)
Maker: Peter Dollond (1731 - 1820)
Cultural Region:
Place of Origin:
Dimensions:
object glass micrometer: 10.4 x 22.6 x 4 cm (4 1/8 x 8 7/8 x 1 9/16 in.)
box: 5.6 x 25.4 x 11.8 cm (2 3/16 x 10 x 4 5/8 in.)
Accessories: Fitted mahogany box
Bibliography:
The Apparatus of Science at Harvard, 1765-1800
Description:
This split object glass micrometer (or heliometer) is made of brass. It is designed to fit on the end of the Cassegrain reflector by James Short (0053) and was sold together with it. There are also holes on the sleeve of the micrometer, where screws could be inserted to fix it to the telescope tube into whose end it slid. These holes match up with those on the telescope.
The longitudinal motion of the objective halves are controlled by dials and a knurled screw. Turning the screw moves (a) a pointer on a scale concentric to it; and (b) a scale in a window. Scale (a) is divided [0]-5-10-15-20 [=0], every 1 unit. Scale (b) is divided [0]-5-25 [=0], every 1 unit.
Another screw (its key missing) rotates the optical assembly around the tube axis. A scale on the circular limb is divided 90°-0°-90°, every 1°.
The original mahogany box is fitted to hold the instrument and has a sliding cover. The box lid has black stamped numerals: 9 142.
The longitudinal motion of the objective halves are controlled by dials and a knurled screw. Turning the screw moves (a) a pointer on a scale concentric to it; and (b) a scale in a window. Scale (a) is divided [0]-5-10-15-20 [=0], every 1 unit. Scale (b) is divided [0]-5-25 [=0], every 1 unit.
Another screw (its key missing) rotates the optical assembly around the tube axis. A scale on the circular limb is divided 90°-0°-90°, every 1°.
The original mahogany box is fitted to hold the instrument and has a sliding cover. The box lid has black stamped numerals: 9 142.
In Collection(s)
Signedunsigned
FunctionThe split object glass micrometer is an instrument for determining the angular separation of two celestial objects, such as two stars. It is sometimes called a heliometer, because it can also be used for determining the diameter of the sun. These micrometers were used to make measurements of Venus's path across the sun's disk during the Transits of Venus in 1761 and 1769.
Historical AttributesThis object glass micrometer was shipped to Harvard from London in August 1765 as part of the first batch of instruments to replace those lost in the Harvard Hall fire of 1764. It was described on the bill of lading as "one Object Glass Micrometer of 21 1/2 feet focus" and was matched to "one Reflecting Telescope of 12. Inches focal length by James Short." Each cost £14.14.0.
John Winthrop used the pair to observe the Transit of Venus in 1769 from Cambridge, Massachusetts.
John Winthrop used the pair to observe the Transit of Venus in 1769 from Cambridge, Massachusetts.
Published ReferencesDavid P. Wheatland, The Apparatus of Science at Harvard, 1765-1800 (Cambridge: Harvard University Press, 1968), 30-31.
Rolf Willach, on object glass micrometers in Journal of the Antique Telescope Society.
Rolf Willach, "List of Extant Reflecting Telescopes Made by James Short," i>Journal of the Antique Telescope Society, no. 29 (Fall 2007): 11-22, no. 102.