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

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

First edition of the founding work on photometry.

“His interest in the measurement of light dates from about 1721, when J.J. d’Ortous de Mairan proposed a problem that necessitated a knowledge of the relative amount of light from the sun at two altitudes. Bouguer succeeded in making such a measurement of the light from the full moon on 23 November 1725, by comparing it with that of a candle. 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 Essay d’optique… 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 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” (DSB).

“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).

The three engraved plates depict the experiments of the author.

A very good copy.