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"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

Differential Lens

"Differential Lens.—Horizontal divergence may be obtained to any required amount by varying the radius…

"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

Differential Lens

"Differential Lens.—Horizontal divergence may be obtained to any required amount by varying the radius…

"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

Differential Lens

"Differential Lens.—Horizontal divergence may be obtained to any required amount by varying the radius…

"A representation of the manner in which the image is formed in the eye, when the cornea or crystalline lens is flattened. The perfect image would be produced at m m, behind the retina, and, of course, beyond the point necessary to secure perfect vision." —Wells, 1857

Flattened Eye

"A representation of the manner in which the image is formed in the eye, when the cornea or crystalline…

An illustration of magnetic curves, "the name given to those curves in which an infinite number of very needles would arrange themselves when placed round a magnet and at liberty to move round an axis." -Century, 1886

Magnetic Curves

An illustration of magnetic curves, "the name given to those curves in which an infinite number of very…

"When the rays diverge from a point beyond the center of curvature, as B, the focus falls on the same axis, at a distrance from the mirror greater than that of the principal focus, and less than that of the center of curvature." -Avery 1895

Rays Diverging from Beyond the Center of Curvature on a Concave Mirror

"When the rays diverge from a point beyond the center of curvature, as B, the focus falls on the same…

"When the object is at a distance from the mirror somewhat greater than the center of curvature, as beyond C, the image is real, inverted, smaller than the object, and at a distance from the mirror greater than that of the principle focus and less than that of the center of curvature, as between F and C." -Avery 1895

Image Beyond the Curvature of a Concave Mirror

"When the object is at a distance from the mirror somewhat greater than the center of curvature, as…

"A representation of the manner in which the image is formed in the eye of a near-sighted person. The curvature of the cornea, s s, and of the crystalline lens, c c, is so great that the image is formed at m m in advance of the retina." —Wells, 1857

Near-sighted

"A representation of the manner in which the image is formed in the eye of a near-sighted person. The…

"A representation of the manner in which the image is formed upon the retina in the perfect eye. The curvature of the cornea, s s, and of the crystalline lens, c c, is just sufficient to cause the rays of light proceeding from the image, e e, to converge the right focus, m, upon the retina." —Wells, 1857

Perfect Eye

"A representation of the manner in which the image is formed upon the retina in the perfect eye. The…