Thorps replica grating (2 1/8 x 1 3/8-inches) on glass (2 1/2 x 2-inches) of Rowlands metal diffraction grating
Plane diffraction grating with 14,438 lines per inch ruled on Rowland's engine at Johns Hopkins University on a plate prepared at the Astronomical and Physical Instrument Works of J.A. Brashear at Allegheny in 1890, on oxidised brass mount with circular rack and pinion for rotating grating, and three levelling screws Plane diffraction grating with 14 1890