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12mo (160 x 97 mm). 3 folding engraved plates, initial blank, errata and publisher's catalogue at end. (Minor marginal toning to some gutters, occasional spots.) Contemporary calf  (rebacked preserving original spine and endpapers, upper fore-corners worn). Provenance: Ferguson of Raith (armorial bookplate).

 

FIRST EDITION OF THE FIRST TREATISE ON PHOTOMETRY. As the discoverer of a practical method of measuring the intensity of light, Bouguer was the founder of this branch of optics. A youthful prodigy who established himself early in life as the leading French authority on nautical matters, Bouguer merely dabbled in optics as a hobby. Nevertheless, the Essai contains two fundamental contributions to the subject. The first was his method of using the naked eye "not as a meter but as a null indicator, i.e., to establish the equality of brightness of two adjacent surfaces" (DSB), and then applying Kepler's law of inverse squares. His second discovery concerned the transmission of light through transparent surfaces: "In a medium of uniform transparency the light remaining in a collimated beam is an exponential function of the length of its path in the medium. This law was restated by J. H. Lambert in his Photometria (1760) and, perhaps because of the great rarity of copies of Bouguer's Essai, is sometimes unjustifiably referred to as Lambert's law" (op. cit.). Norman 283.