Optician Robert-Aglae Cauchoix (1776-1845) created three of the largest telescopes of his time, including an instrument with an aperture of 13.3 inches and 25-foot focal length, completed in March 1831, the largest objective lens made at that time. The owner of the largest telescope, Edward Joshua Cooper, used the instrument to sketch Halley's comet in 1835 and to view the solar eclipse of May 15, 1836. In the 1870s, the Cauchoix objective showed rays coming off stars, from improperly centered lenses, which had been noted when the lens was new. Another of his instruments was used to observe the 1874 Transit of Venus from Hawaii.
Cauchoix specialized in optics, and fabricated instruments including barometers, spherometers, and micrometers. Although there is some thought that the spherometer was invented by the French optician Laroue, the first spherometer of which there is positive knowledge was devised and named around 1810 by Cauchoix, and made by the French mechanician Nicolas Fortin. Cauchoix's design, a three-legged base supporting a central micrometer screw, was quickly adopted as the basic standard and remains in use to this day. Other Cauchoix instruments include a spherometer housed at the Conservatorie National des Arts et Metiers in Paris that reads to 1/1000 millimeter. Other Cauchoix telescopes are housed at the Smithsonian, including a brass variable angle prism scope signed "Cauchoix a Paris", used at the U.S. Military Academy circa 1830. Cauchoix also invented a 'foot' for a telescope, an example of which was under a Lerebours telescope owned by Napoleon, now at the Paris Observatory.
