Replica concave diffraction grating, area 45 x 15 mm approx., made by sticking a metal foil replica grating on the back of a concave glass disc. Markings scratched on the protective black paint on the back of the disc read 'Thorp's replica of grating / about 15100 lines per / inch / T. T. / 14/11/13'. The disc clips into a square wooden block.