![]() ![]() ![]() However, it proved difficult, if not impossible, to capture and communicate in words the manual skills they had acquired. Short’s specialism placed his work in a tradition that derived from Newton’s Opticks, where the natural philosopher or mathematician might engage in the mechanical process of making mirrors, and a number of prominent astronomers followed this example in the eighteenth century. The instrument maker James Short, whose output was exclusively reflecting telescopes, was a sustained and consistent supporter of the clock and watch maker John Harrison. Non-specific advertisements for reflectors continued beyond amalgamation with the Morin Company in 1963, but disappeared after a subsequent merger with the Société de Recherches et de Perfectionnements Industriels c.1967. Only 125- to 200-mm amateur reflectors were offered in their 19 catalogues. Following Georges’s death in 1906 the company was operated by Paul before being sold to Charles Épry in 1906 who associated with Gustave Jacquelin in 1913. Foucault-style reflecting telescopes were offered by other makers too, including Jules Duboscq, Édouard Lutz and Albert Bardou. In 1903 the Secretan company offered a simplified 125-mm reflector designed specially for members of the Société Astronomique de France, perhaps promoted by Georges’s son Paul. Mailhat became director of the company’s workshops and then founded his own firm. Production appears to have revived after R. Production of silvered-glass reflectors and other scientific instruments languished, and focal ratios slowed. Auguste died that year and the firm was taken over by his cousin Georges Secretan. The Secretan offering of silvered-glass telescopes reached its apogee in 1874 with advertised diameters from 10 to 80 cm. Auguste Secretan associated with Paul and Prosper Henry for mirror figuring. Martin worked with both Eichens and, episodically, the Secretan firm but though able to figure small mirrors he proved incapable of finishing 80- and 120-cm ones begun under Foucault destined for the Toulouse and Paris Observatories. Foucault's pupil Adolphe Martin published some of Foucault’s mirror- and lens-making secrets. Marc died in 1867, followed by Foucault in 1868. ![]() In 1866 the head of the Secretan workshop, Wilhelm Eichens, split from the firm. In 1865, with Marc's son Auguste, Foucault announced a metal-mounted 10-cm alt-az amateur instrument, which soon became available in larger sizes and with equatorial mounts. Foucault predominantly adopted an f/6 focal ratio with a prism secondary close to the prime focus and a microscope-like eyepiece assembly to bring the image to the observer. A number of his instruments are illustrated in physics textbooks of the time. He made several professional reflecting telescopes, culminating in the 80-cm instrument now at the Marseilles Observatory. He also introduced the concept of pouvoir optique, or optical power, to characterize the performance of his mirrors. The errors were then corrected with retouches locales, i.e. As Foucault attempted to make larger diameters he moved from spherical to paraboloidal mirrors and developed tests to determine the errors of the surfaces he was polishing, of which the knife-edge test is the most informative and sensitive. Almost immediately, they began selling 4-and 8-(French)-inch Newtonian telescopes in wooden tubes to amateurs. Léon Foucault developed the silvered-glass reflecting telescope in collaboration with the instrument maker Marc Secretan. ![]()
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