Essai d'Optique, sur la Gradation de la Lumiere

Three folding engraved plates. 12 p.l. (the first leaf is a blank), 164, [4] pp. 8vo, cont. calf (upper joint cracked but firm, upper corners a bit worn), spine gilt, red morocco lettering piece on spine. Paris: C. Jombert, 1729.

First edition of the work which clearly established Bouguer as the discoverer of the first practical way of measuring light, as well as nearly all the photometric theory.

"Bouguer's achievement was to see that the eye could be used, not as a meter but as a null indicator, i.e., to establish the equality of brightness of two adjacent surfaces. He then made use of the law of inverse squares, first clearly set forth by Kepler. In his Essai d'optique sur la gradation de la lumière (1729), he showed how to compare lights in this way; he then went on to deal with the transmission of light through partly transparent substances. 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."-D.S.B., II, p. 343.

Very good copy.