An improved method of making replicas of Rowland Diffraction Gratings. Casts are made from the rulings on speculum metal by covering with solution of soluble gun cotton in amylaeetate and allowing same to dry, after which riley are stripped off under water, placed face down on plane glass and the water dried out under pressure, leaving the lines as air spaces between illtn and glass. Tlley are then sealed up under another glass with a belsam mixture, thus protecting them from inJury.--THE EDITOR.]