Rock Salt Prism


John Tyndall, an Irish scientist working at the Royal Institution in London, reported in the late 1850s that prisms of rock salt (NaCl) might be used to examine the infra-red region of the solar spectrum, but were difficult to produce and polish. Samuel Pierpont Langley (1834-1906), director of the Allegheny Observatory in Pittsburgh, asked several opticians to produce prisms of this sort, and while their results proved disappointing, John A. Brashear (1840-1920), a local and self-taught instrument-maker, solved the problem. At the 1885 meeting of the American Association for the Advancement of Science, Langley talked about “Some Hitherto Unmeasured Wavelengths” of lines in the infra-red region of the solar spectrum; and Brashear talked about “A Practical Method of Producing Accurate Rock Salt Surfaces for Optical Purposes.” The following year, Langley became the Assistant Secretary of the Smithsonian Institution, and established the Smithsonian Astrophysical Observatory.

This rock salt prism came from the Smithsonian Astrophysical Observatory. The "R. B. 1." inscription stands for Russia (the crystal was found in Russian pavilion at the Columbia Exposition held in Chicago in 1893) and Brashear.

Location: Currently not on view

See more items in: Medicine and Science: Physical Sciences


Exhibition Location:

Data Source: National Museum of American History

Id Number: PH.327693Accession Number: 268220Catalog Number: 327693

Object Name: rock salt prism

Physical Description: rock salt (overall material)glass (overall material)Measurements: overall: 17 in x 8 in; 43.18 cm x 20.32 cmoverall; old jar: 17 1/2 in x 9 in; 44.45 cm x 22.86 cmoverall; new jar: 10 1/4 in x 9 in; 26.035 cm x 22.86 cm


Record Id: nmah_1183919

Our collection database is a work in progress. We may update this record based on further research and review. Learn more about our approach to sharing our collection online.

If you would like to know how you can use content on this page, see the Smithsonian's Terms of Use. If you need to request an image for publication or other use, please visit Rights and Reproductions.