Replica plane diffraction grating, area 49 x 36 mm, mounted on glass plate with label reading 'Thorp's Transparent Replica of Rowland's / Metal Diffraction Grating / 14505 lines to the inch', and with 'The original is the property of Dr Common / Thomas Thorp / Whitefield [ . . . . ] / Oct 1906' [one word is indecipherable: reading of the year is also uncertain] written in white ink on the black edging tape.
