MEMOIR OF AUGUSTE BRAVAIS. 163 



others, and no shade of rivahy ever found access to hiin. Always dis- 

 posed to render service and to give counsel, when asked for, he fulfilled 

 his own duties with the most scrupuhjos exactness. His lectures drew 

 a numerous anditory, for they were enlivened by the associations which 

 the variety of his studies and experiences presented without elfort to 

 his mind, and to which Ids vivid imagination gave an endlessly diversi- 

 fied expression. In spite of the aridity sup[)osed to he inherent in 

 nmthematical pursuits, Ids conversation was picturesque and sportive, 

 and oiten heightened by sallies in which science allied itself with poetry. 

 It was never witliout regret, therefore, that his colleagues of Lyons heard 

 him speak of withdrawing. Yet, this ]\I. Bravais was bound to think 

 of, for the publication of the voyage of the Scientific Commission of 

 the xsorth was advancing, and with that would finish the mission with 

 which he had been cliargcd by the minister of marine. To remain at 

 Lyons would have been to renounce his career as an officer in that 

 branch of service, and he sometimes thought of requesting to be sent 

 on some new" voyage. Those who justly saw in him the ideal of the 

 scientific traveler could not forbear from encouraging him to do so, but 

 an unforeseen circumstance put an end to these deliberations. 



Our distinguished colleague, M. Lame, had just relinquished the chair 

 of physics in the Polytechnic School to occupy the place of examiner of 

 graduates. The council, with great unanimity, designated M. Bravais 

 to succeed him. The latter, tberefore, a naval lieutenant, was nonunated 

 to replace M. Lame, chief engineer of mines, in a school which furnishes 

 as ^rell ofliccrs to the marine as engineers to the corps of mines and of 

 civil constructions. 



The preparation of a course so high as that with which he was now 

 charged, gave, for some time, a particular direction to the studies of M. 

 Bravais, and he delayed not to publish several excellent memoirs on 

 atmospheric optics and the molescular constitution of bodies. A year 

 after his ascent of Mont Blanc lie presented to the Academy a memoir 

 on the white rai)d)oiv, which completed in a very happy manner one of 

 the most admirable theories of physics. 



Until these latter ages mankind had seen in the rainbow a sign of 

 hope, without knowing the causes of its appearance. Theodorich, De 

 Dominis, Descartes, had explained its formation by the refractions and 

 reflections undergone, in drojis of rain, by the rays of the sun. Newton 

 had completed this explanation by tiie consideration of the unequal 

 refrangibility of colors ; but none of these eminent physicists, and none 

 of those who after them had been occupied with the details of the 

 phenomenon, had ex])lained in a satisfactory manner the formation of the 

 ichite rainbow^ which is sometimes seen to make its appearance on fogs of 

 little elevation and not far remote from the spectator, with a radius 

 sensibly inferior to that of the ordinary rainbow. It was reserved for 

 M. Bravais to demonstrate that if a cloud is formed of small hollow 

 spheres in which the thickness of the watery envelope is comprised be- 

 tween thirty-eight and fifty-five-hundredths of the radius of the internal 

 vacuum, it must form a white luminous arc of thirty-four to forty degrees 

 of radius, ami consequently not so large as the ordinary rainbow of the 

 first order, whose radius is 42° 20'. In ordinary clouds the envelope of 

 the globules of vesicular vapor is thinner than that required by the 

 theory of the white raiid)ow, whence it results that it is not formed. 

 It is only to be seen on heavy fogs attached to the surface of the land 

 or sea. True, this Avhite rainbow is not one of those phenomena which 

 strongly captivate the imagination ; but it is enough for the honor of 

 our colleague to remark that all physicists and Newton himself had left 



