BOUGUER Pierre Essai d’optique sur la gradation de la lumière.

DSB, II, pp. 343-344 ; DiLaura, 308 ; Norman I, 283.

Édition originale, très rare.

L’ouvrage fondateur de la photométrie, branche de l’optique consacrée à la mesure de l’intensité de la lumière. Dans la première section de l’ouvrage, intitulée “Méthode de mesurer la force de la lumière”, Bouguer expose la loi de variation de l’éclairement ; la deuxième partie, “De la transparence et de l’opacité”, contient l’énoncé de la loi de Bouguer.

“In the latter part of the Essai, Bouguer published the second of his great optical discoveries, often called Bouger’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 related 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. Just before he died, Bouguer completed a much larger book on photometry, the Traité d’optique sur la gradation de la lumière, published posthumously (1760) by his friend the Abbé Nicolas Louis de la Caille” (D.S.B).

“An early treatise on the measurement of light using visual photometry. The distances of two sources that illuminated a split visual field were changed until the field was equally bright; the ratio of the square of the distances gave the relative luminous power of the two sources. This relative photometric method was used to determine the luminous power of candle to that of the sun and other sources. It was also used to determine the reflectivity of surfaces and the absorption of light in air and water, giving Bouger’s exponential law of attenuation” (DiLaura).

Les 3 planches gravées sur cuivre représentent plusieurs croquis et schémas décrivant les expériences optiques de l’auteur.

Très bon exemplaire.