Polishing Rock Salt
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10/31/2006
3:32 PM
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HISTORICAL NOTE
Polishing Rock Salt At the 1885 meeting of the American Association for the Advancement of Science in Ann Arbor, Michigan, Samuel Pierpont Langley (1834–1906)—the astronomer at the Allegheny Observatory in Pittsburgh, pioneer of flight, and soon-tobe second secretary of the Smithsonian Institution—presented a paper entitled “Some Hitherto Unmeasured Wavelengths.” Langley’s astronomical research in Pittsburgh was largely confined to studies of the sun, because the ever-present smoke from the city’s industrial mills blocked the view of more distant stars on most nights. His efforts to measure how much solar radiation was absorbed by the Earth’s atmosphere led him into spectroscopic investigations of sunlight. In his paper, he described the resolution of the Fraunhofer lines—dark lines caused by absorption of radiation by cooler elements in the sun’s outer atmosphere—in the infrared region of the spectrum with previously unhoped-for precision. Indeed, he was able to see the nickel absorption line located between the two sodium lines of the spectrum for the first time. This tremendous advance in the field of infrared spectroscopy was made possible, he said, through the use of rock salt prisms made by the Pittsburgh optician John A. Brashear (1840–1920). The Irish scientist John Tyndall (1820–1893) had earlier established that rock salt (NaCl) would be a valuable material in the investigation of the infrared portion of the spectrum because of its transparency at those wavelengths. But by the 1880s, no one had devised a method of polishing rock salt to optical quality. The softness of rock salt and the tendency of its surface to dissolve in absorbed moisture made the task difficult. Indeed, many scientists said that rock salt could never be polished well enough to resolve Fraunhofer lines. Not easily discouraged, Langley turned to the best opticians of his day to solve the problem. He gave a French optician named Hoffman the first chance, but Hoffman’s prisms were able only to resolve the major Fraunhofer lines at best. In a paper entitled “A Practical Method of Producing Accurate Rock Salt Surfaces for Optical Purposes,” presented right after Langley’s paper at the Ann Arbor meeting, Brashear attributed the low quality of Hoffman’s prisms to the process of rubbing the rock salt crystal on a broadcloth surface to polish it; this led
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to rounding of the corners of the crystal, causing “overcorrection.” “This is fatal to good results in any optical surface,” Brashear noted. Langley next approached George Clark of Alvan Clark & Sons, a noted telescope lens maker in Cambridgeport, Massachusetts. Clark succeeded in producing one excellent rock salt prism for Langley but declined to supply more. Clark later told Brashear that he had had to wipe the finished rock salt surface on his palm or arm to remove excess moisture, but more often than not, this ruined the planarity of the polished surface.
…many scientists said that rock salt could never be polished well enough to reso
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