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Barlow, Peter

lens magnification mathematics iron

(1776–1862) British mathematician.

Self-educated, Barlow taught mathematics at the Royal Military College, Woolwich, from 1801. Of his books on mathematics, the best known is Barlow’s Tables (1814) which gives the factors, squares, cubes, square and cube roots, reciprocals and hyperbolic logarithms of all integers from 1 to 10 000. Remarkably accurate, it was familiar to generations of students and was in print until the 1950s. He also worked on magnetism and devised a method for correcting ships’ compasses for deviation due to iron in the ship’s structure, by use of an iron plate suitably positioned. The ‘Barlow lens’ is a negative achromatic combination of flint and crown glass used to produce magnification of a photographic or telescopic image. Used in addition to a camera lens as a ‘telescopic converter’ it will give a magnification of 2× or 3× with acceptably little aberration. A similar use of a Barlow lens is between the objective and eyepiece of a telescope, where again it increases magnification by extending the focal length of the main lens.

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