Hall, Edwin Herbert, 1855-1938 · New York, H. Holt and company · leaf 149
136 PHYSICS. Lenses. 136. Shapes of Lenses.—A lens is, usually, a piece ofglass whose two faces are parts of spherical surfaces. Sometimes there is a cylindrical surface between the twospherical faces. Fig. 101 shows various lenses as they would look if cutthrough the middle. … Fig. 101. Lenses are classed as convex, or converging, and concave,or diverging. Convex lenses are all thicker in the middlethan at the margin, and cause parallel light-rays to con-verge, as in Fig. 102. Concave lenses are thinner in the
Physics
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