The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge . 393. Parallel rays passing through a doubleconcave lens may also be made to diverge. Thus. the rays a, b, c, &c,,after passing through the doubleconcave lens A, will go on in a diverging state, thesame as if the lens were taken away and the rayshad proceeded from a point F in thevirtual focus or centre of concavity of the lens. 394. The manner in wliich the foci of lensesof different curves are calculated, and how thefoc
The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge . 393. Parallel rays passing through a doubleconcave lens may also be made to diverge. Thus. the rays a, b, c, &c,,after passing through the doubleconcave lens A, will go on in a diverging state, thesame as if the lens were taken away and the rayshad proceeded from a point F in thevirtual focus or centre of concavity of the lens. 394. The manner in wliich the foci of lensesof different curves are calculated, and how thefoci of combined lenses may be obtained, are asfollows :—When the lenses are made of plate-glass, the focal distance is nearly the diameter ofthe sphere from which we may suppose a plano-convex lens to be cut, or it is equal to twice theradius of the circle that forms the convex surfaceof the lens. For example, if *he globe of glassis one inch in diameter, and a portion is cut offto form a plano-convex lens, the focus will beone inch, or twice the radius of the circle. Ifthe lens is double convex, the focus will be equalto the radius, or half the diameter. When thelens is crossed, or unequally convex, the focallength will be twice the product of
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