. The principles of physics. iderable distance from a window, and regulating thedistance so that a distinct image of the window will be pro- jected upon (1 opposite 11, as in 1 .269. The 1 il length is IIf distance I n the opti- 1 center of 1 lens to the ter of the ^^^r^:?=^:7mm image on the paper. The shorter the focal length the more powerful is the lens ;that is, the more quickly are the parallel rays that traversedifferent parts of the lens brought to cross one auother. If the paper be kept at the princiiaal focus for a short time,it will take lire. The reason is apparent why convex lenses
. The principles of physics. iderable distance from a window, and regulating thedistance so that a distinct image of the window will be pro- jected upon (1 opposite 11, as in 1 .269. The 1 il length is IIf distance I n the opti- 1 center of 1 lens to the ter of the ^^^r^:?=^:7mm image on the paper. The shorter the focal length the more powerful is the lens ;that is, the more quickly are the parallel rays that traversedifferent parts of the lens brought to cross one auother. If the paper be kept at the princiiaal focus for a short time,it will take lire. The reason is apparent why convex lenses aresometimes calledburning pencil of rays,emitted from theprincipal focus F(Fig. 268) as aluminous point, be-comes parallel onemerging from a convex lens. If the rays emanate from apoint nearer the lens, they diverge after egress, but thedivergence is less than before; if from a point beyond theprincipal focus, the rays are rendered convergent. A concavelens causes parallel incident rays to diverge as if they came. Fig. 270. CONJUGATE FOCI. 361
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1895
