Speculum mirror

    Date
    ca.1723
    Creator
    John Hadley (1682 - 1744, British) , Natural Philosopher
    Object type
    Archive reference number
    Material
    Subject
    Content object
    Description
    A speculum mirror turned to the left [as viewed], in a wooden cell, with screw-in handle and three eyepieces positioned in front. Intended for a Newtonian reflecting telescope.

    Speculum written about at length in John Hadley's paper, 'An account of a catadioptrick telescope [...]', Philosophical Transactions of the Royal Society, vol. 32, no. 376 (1723). Descriptions include: 'THE Instrument consists of a metal line Speculum about six Inches in Diameter. The Radius of the Sphere, on which its concave Surface was ground, is ten Feet, five Inches and one quarter, and consequently its focal Length is 62 nches. The Back has a hollow Screw made at its Centre, to receive the End of a Handle, which is screw’d on, whenever the Metal is to be moved, in order to avoid sullying its polish'd Surface by handling.'

    John Hadley (1682-1744) British natural philosopher and mathematician was elected a Fellow of the Royal Society in 1717. Unlike his predecessors in the evolution of the reflecting telescope, namely James Gregorie and Isaac Newton, Hadley developed ways to make parabolic speculum metal mirrors, allowing for higher definition and greater image clarity in his telescopes.

    James Gregorie (1638-1675) Scottish mathematician and astronomer was elected a Fellow of the Royal Society in 1668. In 'Optica Promota' (1663) Gregorie pointed out that a reflecting telescope with a mirror that was parabolic would correct spherical aberration seen in refracting telescopes, however, he could not find a craftsperson capable of constructing one.

    Sir Isaac Newton (1642-1727) natural philosopher and mathematician was elected a Fellow of the Royal Society in 1672. Newton was also confronted with the problem of fabricating the complex parabolic shape and chose a spherical shape for his reflecting telescope mirror in the late 17th century.
    Object history
    Exact provenance unknown. Mirror and telescope apparatus first shown to the Royal Society in 1721 and likely presented to the Society at a subsequent date by Hadley himself. Loaned to the Science Museum in 1932 and returned to the Royal Society in 2019.
    Associated place
    <The World>
       > Europe
          > United Kingdom
    Credit
    ©The Royal Society
    Image number
    RS.18183
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