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“Throw a sunbeam through a very small opening in the shutter of a darkened room. Receive the beam upon a convex lens of short focal length, placing a pieve of red glass between the aperture and the lens. Place and opaque screen with a sharp edge beyond the focal distance of the lens, as at a, so as to cut off the lower part of the cone of homogeneous light, and project the upper part thereof upon a screen at b.” -Avery 1895


Elroy M. Avery School Physics (New York: Sheldon and Company, 1895) 394


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