BOUGUER, [Pierre].

A Paris: Chez Claude Jombert… .1729

12mo, 12 leaves (including the first blank), 164 pages, 2 leaves, and 3 folding engraved plates. Contemporary mottled calf (upper joint cracked but firm, tips of upper corners worn), spine gilt in compartments, red morocco label, red edges, marbled endpapers.

FIRST EDITION of the book which “clearly establishes Bouguer as the discoverer of the first practical way of measuring light, as well as nearly all the usual photometric theory. In particular it sets out the exponential law of attenuation of light in transparent media, a law which is frequently attributed to Lambert or even to Beer” (Middleton). “In the latter part of the Essai, Bouguer published the second of his great optical discoveries, often called Bouguer’s law: 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” (Middleton in DSB). The Essai is considerably rarer than the extended version which was posthumously published by La Caille under the title Traité d’Optique in 1760. As well as W.E. Knowles Middleton’s article on Bouguer in the DSB, see also his “Note on the Invention of Photometry” in Amer. J. of Physics, 31 (1963), pp. 177–181.

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