It would have been used to test the flatness of an unknown surface. Lay the flat face down on top of the unknown surface, illuminate with monochromatic light (sodium or filtered mercury), and a series of interference fringes, much like a contour map, will appear
Industrial strength marker perfect for tough marking jobs Oversized felt chisel tip draws extra wide, highly visible lines on corrugate, wood, metal, foil, stone, plastic and leather Specially formulated to mark on both wet and oily surfaces Durable aluminum body and felt
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