Accession No

2110

Brief Description

diffraction grating, reflecting, Rowland-type, by John A. Brashear, U.S.A., 1889

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

1889

Latest Date

1889

Inscription Date

1889

Material

wood; metal (brass, speculum metal); cloth (felt)

Dimensions

plate 52 x 52 x 8mm; diameter polished surface 50mm; ruled surface 44 x 21mm

Special Collection

Provenance

Transferred from Institute of Astronomy, University of Cambridge, Cambridge, England, 10/1975.

Inscription

‘GRATING J.K.M. Gift’ (outside of box)
‘Mr Pillson [?] Paris Best Quality’ (inside of box)
‘Ruled on Rowlands Engine John Hopkins University Baltimore M.d. 1889 R = 52 [not equal to sign]’ (grating)
‘Plate polished and erected at the Astronomical instrument works of J.A. Brashear Allegheny Pa U.S.A.’ (grating)

Description Notes

Diffraction grating, reflecting, Rowland-type, by J.A. Brashear, U.S.A., 1889.

Rowland reflection grating; square speculum metal plate with raised circular polished surface, containing rectangular ruled surface.
Fitted wooden box with brass hinges and hook fastener.

Carries the following inscriptions:

‘GRATING J.K.M. Gift’ (outside of box)
‘Mr Pillson [?] Paris Best Quality’ (inside of box)
‘Ruled on Rowlands Engine John Hopkins University Baltimore M.d. 1889 R = 52 [not equal to sign]’ (grating)
‘Plate polished and erected at the Astronomical instrument works of J.A. Brashear Allegheny Pa U.S.A.’ (grating)

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 Institute of Astronomy at the University of Cambridge, but the Whipple Museum also has gratings from the Cavendish laboratory and R.S. Whipple’s own collection.

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:40370