There is a gap of nearly twenty years between Merton's scientific papers of 1928 and 1947. In this interval he was busy in the laboratory and was taking out patents for his inventions. Diffraction gratings were one of his lifelong interests and here his inventive genius best showed itself. The rarity and expense of good diffraction gratings led him to devise, in 1935, a method of copying them without loss of optical quality, by applying a thin layer of a cellulose ester solution to an original plane grating. When the solvent had evaporated he detached this pellicle and applied its grooved surface to a moist gelatine film on a glass plate. When dry, the gelatine bore a faithful record of the original rulings.