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The production of a plane surface, as illustrated by specula from some early Newtonian telescopes

THE construction of accurate paraboloidal mirrors for reflecting telescopes has been the subject of much attention from physicists, practical opticians, and historians of science. However, the paraboloidal primary must be combined with a secondary mirror to give any form of working telescope other than the Herschelian design, and the quality of this secondary is every bit as important to the definition of the final image. In the Newtonian reflector the secondary mirror takes the form of a plane surface of elliptical outline. Historical specula therefore provide a practical record of attempts to achieve a perfectly flat solid surface— an undertaking which has often proved more difficult than making mirrors of a curved figure. Plane surfaces were subsequently required in many optical instruments besides telescopes, such as prisms and diffraction gratings. Then, in the early decades of the nineteenth century, another field of application was created when Maudslay, Whitworth and others realized the need for accurately flat surfaces in precision engineering and toolmaking. To test their products and ensure interchangeability they had to produce plane surfaces upon cast iron ‘surface plates’ and the hardened steel anvils of screw micrometers.