# Focal-lengths meaning

*focal length.*

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Plural form of *focal length.*

noun

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A volume entitled Opera posthuma (Leiden, 1703) contained his "Dioptrica," in which the ratio between the respective focal lengths of object-glass and eye-glass is given as the measure of magnifying power, together with the shorter essays De vitris figurandis, De corona et parheliis, &c. An early tract De ratiociniis tin ludo aleae, printed in 16J7 with Schooten's Exercitationes mathematicae, is notable as one of the first formal treatises on the theory of probabilities; nor should his investigations of the properties of the cissoid, logarithmic and catenary curves be left unnoticed.

The magnifying power of the telescope is = Ff /ex, where F and f are respectively the focal lengths of the large and the small mirror, e the focal length of the eye-piece, and x the distance between the principal foci of the two mirrors (=Ff in the diagram) when the instrument is in adjustment for viewing distant objects.

(This explains the gigantic focal lengths in vogue before the discovery of achromatism.) Examples.

486'2 434' 1 405.1 kï¿½, and the focal lengths are made equal for the lines C and F.

Should there be in two lenses in contact the same focal lengths for three colours a, b, and c, i.e.