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Lens (Transparent substance usually)

a transparent substance, usually glass, surrounded by curved surfaces so formed that rays of light passing through it are made to change their direction, and to magnify or diminish objects at a certain distance. Lenses are double convex, or convex on both sides; double concave, or concave on both sides; with one side plane and the other con- plano convex, or plano concave, i. e. with one side plane and the other conand concave on the other. If the convexity be greater than the concavity, or if the two surfaces would meet if produced, the lens is called a meniscus; and if the two surfaces would meet if proif the concavity be greater than the convexity, the lens is termed concavo-con-

Every system of lenses, however vex. complicated and whatever be the mutual distances of the lenses, will, if the whole complicated and whatever be the mutual be centered on a common axis, produce a real image somewhere in front of the last refracting surface, or else will appear to produce a virtual image somewhere behind. The rays on being traced where behind. The rays on being traced a telescope-undergo numerous deviations: ultimately there is a deviation which might have been equally produced by an equivalent lens; equivalent in the sense of producing an equal ultimate de- by an equivalent lens; equivalent in the which might have been equally produced viation, for the image is not formed in lens" would have formed it in. The sys- the same place as the "single equivalent lens" would have formed it in. The syslent in its action to a simple lens plus a determinate shifting of the focus. lent in its action to a simple lens plus a Hence a simple lens-diagram, modified so as to represent the shifting, will represent the aggregate effect of the most complex system of lenses. See OBSERV- ATORY.