Accession No
1913
Brief Description
diffraction grating, reflecting, Rowland-type, used at the Cavendish Laboratory, by John A. Brashear, U.S.A., 1887
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
1887
Latest Date
1887
Inscription Date
1887
Material
metal; brass; speculum metal; glass
Dimensions
diameter of base 119mm; height 102mm
Special Collection
Provenance
Transferred from the Cavendish Laboratory, University of Cambridge in 07/1974. It is recorded in the Cavendish Laboratory's 1894 alphabetical catalogue of apparatus on p.83.
Inscription
‘Ruled on Rowlands Engine
John Hopkins Univty
14438 lines to 1 in
1887
Every one hundredth
line extended on
one side
Plate Polished & corrected by J.A. Brashear
Allegheny Pa
usa
AE:DL’ (scratched on polished surface)
‘L 56’ (painted on base).
Description Notes
Diffraction grating, reflecting, Rowland-type, by J. A. Brashear, U.S.A., 1887.
Square speculum metal plate, with circular raised polished surface containing rectilinear ruled surface. Mounted behind glass in part-oxidised brass frame. Glass retained by three oxidised brass clamps, secured by three knurled screws. The whole set in a a flat brass ring with three levelling screws.
Inscribed with the following:
‘Ruled on Rowlands Engine
John Hopkins Univty
14438 lines to 1 in
1887
Every one hundredth
line extended on
one side
Plate Polished & corrected by J.A. Brashear
Allegheny Pa
usa
AE:DL’ (scratched on polished surface)
‘L 56’ (painted on base).
Condition good; complete.
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 the Cavendish laboratory, but the Whipple Museum also has gratings from R.S. Whipple’s own collection 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:40438
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