A manual of photographic chemistry, theoretical and practical . ive powers, such figures maybe represented, nearly, by others bounded by straight lines, andthus it becomes evident that a biconvex lens tends to condenserays of light to a point, and a biconcave to scatter them. Amensicus combines both actions, but the rays are eventually benttogether, the convex curve of a meniscus lens being alwaysgreater than the convave. The Foci of Lenses.—It has been shown that convex lensestend to condense rays of light and bring them together to apoint. This point is termed the focus of the lens. 330 OPTI
A manual of photographic chemistry, theoretical and practical . ive powers, such figures maybe represented, nearly, by others bounded by straight lines, andthus it becomes evident that a biconvex lens tends to condenserays of light to a point, and a biconcave to scatter them. Amensicus combines both actions, but the rays are eventually benttogether, the convex curve of a meniscus lens being alwaysgreater than the convave. The Foci of Lenses.—It has been shown that convex lensestend to condense rays of light and bring them together to apoint. This point is termed the focus of the lens. 330 OPTICS OF PHOTOGRAPHY. The following laws as regards the focus may be laid down :—That rays of light which are pursuing a parallel course at thetime they enter the lens are brought to a focus at a point nearerto the lens than diverging rays. The rays proceeding fromvery distant objects are parallel; those from objects near athand diverge. The suns rays are always parallel, and the di-vergence of the others becomes greater as the distance from thelens is Fig. 21. Fig. 22. Fig. 23. The focus of a lens for parallel rays is termed the principalfocus, and is not subject to variation; this is the point re-ferred to when the focal length of a lens is spoken of. Whenthe rays are not parallel, but diverge from a point, that pointis associated with the focus, and the two are termed conjugatefoci.
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