2 Large Precision laboratory diffraction gratings for optical experiments possibly for use with astronomy, the large diffraction gratings both by Bausch & Lomb Rochester New York on large silvered glass blocks in metal mounts, the grating window 13.5cm x 11cm

Note for a similar diffraction grating see https://airandspace.si.edu/collection-objects/diffraction-grating-prime-focus-spectrograph/nasm_A19980111000