ROCHON, ALEXIS-MARIE, member of the Royal Academy of Sciences, was born at Brest on the 21st Feb. 1741. From his residence at Brest, in the midst of ships and sailors, he acquired an earl); taste for the objects of naval science. In 1765, he was named cor responding member of the Academy of Sciences. Some time afterwards he was appointed astronomer to the navy, and in that capacity he performed a voyage to Morocco, in 1767 ; and immediately upon his return to Europe, he set out for the East Indies, in a vessel com manded by his friend and relation, M. de Fromelin ; and during that voyage, in 1769, he determined the position of the islands and rocks between the Mauritius and the Indian coast.
Upon his return from the Mauritius, in 1772, with M. de Poivre, Rochon brought from Madagascar the finest crystals of quartz that had at that time been seen. This accidental circumstance led him to ascertain its double refraction, and by the combination of two prisms, cut in a particular manner, he measured the double re fraction, so as to apply it to the purpose of a double image micrometer. This idea he first applied to a coming-up glass, for ascertaining whether a ship was approaching to, or receding from the observer. A full account of
the invention was published at Paris in 1807, in a pam phlet entitled, Menzoire sur le Micrometre de Cristal de Roche, pour la mesure des distances et des grandeurs. In the year 1812, he laid before the Institute an account of an improvement upon the micrometer ; and in the same year he read a memoir, recommending the substi tution of mica in place of glass, in the different light houses of France. He was led to this idea, from the circumstance of a flock of wild ducks having, in a dread ful tempest, thrown themselves against the glass frame, broke the panes of glass, and caused the light to be ex tinguished. These panes were immediately replaced by Rochon with squares of mica.
M. Rochon had the merit of proposing the canal from Brest to Nantes across Brittany. He was made a mem ber of the legion of honour by Bonaparte ; and he died in the 77th year of his age at Paris, and was buried on the 7th April, 1817. In 1783, Rochon published a work, entitled Recueil de Itlemoires sur la Mechanique et la Physique. See Dr. Brewster's Treatise on New Phi losophical Instruments, p. 188, 189, note. See also our articles MICROMETER, and Or•tcs.