Accession No
0484
Brief Description
diffraction grating, reflecting, Rowland-type, by John A. Brashear, U.S.A., 1891
Origin
U.S.A.; Massachussetts; Baltimore; and U.S.A.; Pennsylvania; Allegheny
Maker
Brashear, John A. [polished and figured] Theodore C. Schneider [ruled using Rowland's engine]
Class
optical
Earliest Date
1891
Latest Date
1891
Inscription Date
1891
Material
metal; speculum metal; wood; brass; felt
Dimensions
plate 36 x 36 x 6mm; diameter polished surface 35mm; ruled surface 27 x 20mm
Special Collection
Robert Whipple collection
Provenance
Purchased by Robert Stewart Whipple from T.H. Court on 05/04/1929.
Inscription
‘Ruled on Rowlands Engine Baltimore 1891
14438 lines to one inch
Plate prepared at the Astronomical + Physical Instrument works of J.A. Brashear Allegheny Pa
AE’ (scratched on grating)
Description Notes
Diffraction grating, reflecting, Rowland-type, by J. A. Brashear, U.S.A., 1891.
Rowland diffraction (reflection) grating. Square speculum metal plate, with circular raised polished surface, containing rectangular ruled surface. Fitted wooden box with brass hinges and hook fastener.
Inscribed with the following:
‘Ruled on Rowlands Engine Baltimore 1891
14438 lines to one inch
Plate prepared at the Astronomical + Physical Instrument works of J.A. Brashear Allegheny Pa
AE’ (scratched on grating)
Condition good.
References
Events
Description
Diffraction grating
A diffraction grating consists of very closely spaced lines on a reflecting surface (the line ruling process must be very precise). The lines disperse (separate) light into its different wavelengths, which are seen as different colours, much like a prism does.
This diffraction grating was ruled at John Hopkins University, Baltimore, USA, on the engine designed by the Professor of Physics H.A. Rowland. Rowland supplied his gratings at cost price to physicists throughout the world. This example comes from R.S. Whipple’s own collection, but the Whipple Museum also has gratings from the Cavendish laboratory and the Institute of Astronomy at the University of Cambridge.
Italian physicist Grimaldi first saw that the edges of shadows are not completely sharp, but have fringes. Later, in 1785, when multiple edges were placed next to one another to form a grating, was the full importance of this observation shown.
01/08/2006
Created by: Saffron Clackson; updated by Ruth Horry on 01/08/2006
FM:40373
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