Differential Lens

“Differential Lens.—Horizontal divergence may be obtained to any required amount by varying the radius of curvature of the inner face of an annular lens. The outer face is the same as that of an ordinary annular lens, which the other face, though straight in the vertical, is ground to the required curve in the horizontal plane. The rays f’fc falling upon the lens x converge to the vertical focal plane ff and afterwards diverge through the smaller horizontal angle x’f’x’, and so for any other case.” —The Encyclopedia Britannica, 1910

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