Rowland concave diffraction grating in stand

Date1897

Inventory Number2037

ClassificationDiffraction Grating

Subject

Maker Henry A. Rowland 1848 - 1901

Maker American (Johns Hopkins University)

Maker John A. Brashear 1840 - 1920

User Department of Physics, Harvard University founded 1884

User Department of Engineering Sciences, Harvard University founded circa 1918

Cultural Region

Place of Origin

Dimensions18.8 × 11.5 × 7.5 cm (7 3/8 × 4 1/2 × 2 15/16 in.)

Material

DescriptionThe diffraction grating, made of solid speculum metal, is divided with 14,438 lines per inch. The radius of curvature is approximately 183 cm. It is mounted in a wooden stand painted black. The grating can be coarsly adjusted. The letter "H" (meaning Harvard College) has been painted in orange at the back of the plate.

Signedon front: Plate prepared at the Astronomical and / Physical Instrument works of / John Brashear Allegheny Pa / 14438 lines to 1'in 568 to 1 mm on front: Ruled on Rowland's Engine / Johns Hopkins University / Baltimore Maryland / R = 6ft ± 1897

FunctionA diffraction grating is a glass or mirror that has regularly spaced microscopic interruptions to the passage of light (lines) such that light passing through or reflected by it experiences interference phenomena, magnifying its magnitude at some angles and cancelling it at others. As this effect depends on the wavelength of light, a diffraction grating is an effective method of decomposing light into its constituent wavelengths, that will be transmitted at different angles (short wavelengths -- bluer -- are deflected less than longer -- redder -- ones). If one knows the line density (lines per inch) and the angle of deflection (with a spectrometer), one can deduce the exact wavelengths.

Diffraction gratings like this one were often used in astronomy to produce spectra of celestial objects. Begun in 1882, Rowland's gratings were more than an order of magnitude larger and more accurate than any previous one. With these, for instance, it became possible to measure the wavelength of light emitted by atoms to unprecedented accuracy. Rowland himself was able to draw a new and more accurate map of the solar spectrum.

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ProvenanceDepartment of Physics, Harvard University, circa 1897; used for teaching optics Harvard College students in the early 20th century; found in Optics Section of Physics Department, then in the Department of Engineering's Pierce Hall, Harvard University.; transferred to CHSI, 1961.