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Inventory no. 22318 - Former Display Label

English MICROMETER EYEPIECE for a TELESCOPE
c.1710

Lignum vitae, brass, and silvered brass. Unsigned and undated.

This is probably the earliest surviving micrometer eyepiece for a telescope. It was used with the large refractor in the Orrery Collection (no. 16), displayed on top of this case.

The evidence of a letter from William Crabtree (1610-?1644) to Jeremiah Horrocks (?1617-1641) shows that the micrometer eye-piece was invented by William Gascoigne (?1612-1644) as early as 1639. It did not become widely known, however, and the device was reinvented by Adrien Auzout (1622-1691), who announced his discovery in 1666 (Philosophical Transactions of the Royal Society, 1667, no. 21). Richard Townley (fl. 1660-1705) pointed out Gascoigne's priority, and demonstrated an improved micrometer to the Royal Society in 1667.

Measurements are made by turning a screw which moves two pointers symmetrically disposed with respect to the centre of the telescope tube. At the zero position the edges of the pointers are in contact. Turning the screw moves the pointers in opposite directions until the edges are tangential to a planet under observation, or until the edges appear to cut two stars whose angular separation is required. The traverse of the pointers is recorded by two scales. It is likely that Gascoigne's original instrument had solid pointers. Auzout employed wires, which make alignment easier. The micrometer shown here is fitted with wires.

[Orrery Collection no. 6]
Lent by Christ Church

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