Pierre Bouguer. Essai d’optique sur la gradation de la lumière. (Les maîtres de la pensée scientifique, publiée par les soins de Maurice Solovine). Paris. Gauthier-Villars. 1921. XX + 132 pp.

Pierre Bouguer. An Essay on the Gradation of Light.

The doctrine of the strength of light historically lagged considerably in its development behind other areas of optics. In the treatises of Newton and Huygens, complex questions of optics were posed and to a considerable extent solved: the decomposition of light, the periodicity of luminous phenomena, diffraction, double refraction, polarization; yet almost nothing was said about the strength of light. The brightness or intensity of light was evaluated in an entirely subjective manner. The problem was first truly posed and brilliantly solved only after Newton’s death by Pierre Bouguer (1698–1758). It is difficult to grasp the basis of that persistent historical injustice by which, down to our own time, laws formulated with complete clarity and precision by Bouguer have been associated with the names of other authors (Beer’s law, Lambert’s laws, etc.). Even in Mach’s most recent history of optics (E. Mach, Die Prinzipien der physikalischen Optik, 1921), Bouguer is allotted very little space, and his principal achievements are not mentioned. Meanwhile Bouguer provided all the principles of photometry that we still use unchanged to this day; he formulated mathematically (in differential and integral form) the fundamental law of the absorption of light as a function of brightness, layer thickness, and concentration; he indicated various methods of photometry and means of quantitatively weakening light (distance, diaphragming, absorbing media); and finally he solved a number of fundamental problems of photometry: determining the absorption of the atmosphere, the distribution of the brightness of light over the solar disk, comparing the brightness of sunlight and moonlight, determining the brightness of reflected light as a function of the angle of incidence

etc. In connection with problems of photometry, Bouguer also dealt with many questions of physiological optics (the “Weber–Fechner law,” successive images, etc.). Bouguer’s exceptional caution in drawing conclusions of every kind, the thoroughness of his experimentation, and the elegant geometrical treatment of mathematical questions distinguish all his works. In any case, Bouguer’s name in the history of optics should stand alongside the names of Newton and Huygens.

The new edition of Bouguer’s Essai, which appeared in 1729, two years after Newton’s death, and which, as far as we know, has not been reprinted since, should be especially gratifying to all those interested in the history of science. Unfortunately, the principles on which the edition of the new French series of “classics of scientific thought” is based (so far 13 numbers have appeared and 33 are planned for publication) do not satisfy the elementary requirements that may be made of modern editions of classical works. Editions of this kind should be either an exact reprint, or a translation of one definite edition, preserving as far as possible all the features of the original. Since the author being published is comparatively little known, biographical data should be appended and, most importantly, a bibliographical index. A reissue is, of course, intended for a comparatively broad circle of readers—scholars who are not specialists in the history of science, or students—and therefore a historical-scientific commentary compiled by a specialist is indispensable. In the edition under review of Bouguer’s Essai, apart from the text itself, there is only a very brief biographical note and a small table for converting old French measures into the metric system. There are no notes at all, although in the edition of the Essai in particular one cannot do without them. In 1760, two years after Bouguer’s death, the Abbé de la Caille (de la Caille) published Bouguer’s large optical treatise, prepared by the latter for the press and constituting a complete, greatly expanded reworking of the Essai (in the treatise VI + 368 pages, large format). The experimental part was considerably enlarged, some hypothetical passages were removed, and the whole exposition was given an impeccably experimental-mathematical character. Bouguer’s treatise is, in this respect, a classic model of a physics book without hypotheses. In any case, it was precisely the treatise that should have been reprinted; in a reissue of the Essai, however, it was necessary to indicate the principal changes. In the edition under review, the existence of the Treatise is merely noted, without any indication of its special features.

Of course, the editorial shortcomings of the edition do not diminish the fact that Bouguer’s hitherto inaccessible Essai has become available to many. One may hope that, in connection with this, interest will be awakened in Bouguer’s personality and in his scientific legacy.

S. Vavilov.