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only about ${\tfrac {1}{15}}$ mm. broad. A recess has first been made in the board, just where the slit lies, so that the latter is free on both sides. The two plates being first brought within a small distance of each other, some powdered calcium sulphide is packed in between them, after which they are pressed against each other, and maintained with screws, which keep them in contact with the board. The compressed sulphide remains in the slit, and is exposed to sunlight, after the excess has been removed. An extremely narrow phosphorescent slit is thus obtained.