220 M. Arago's Historical Eloge of Joseph Fourier. 



naces through a piece of common glass, expecting, by means of 

 this contrivance, to intercept the heat which would have burned 

 their eyes. 



In all the experimental sciences, the periods of brilliant pro- 

 gress are almost always separated from each other by long in- 

 tervals of nearly perfect repose. Thus, after Mariotte, more 

 than a century passes without history having to record any new 

 property of radiant heat. Afterwards, and step by step, there 

 are found in the solar light, non-luminous calorific rays, whose 

 existence could not have been proved unless by the thermome- 

 ter, and which can be completely separated from the luminous 

 rays by means of the prism ; it is discovered, in reference to 

 terrestrial bodies, that the emission of calorific rays, and, conse- 

 quently, the cooling of these bodies, is considerably lessened by 

 polishing their surfaces ; and that the colour, the nature, and the 

 thickness of the covering with which their surfaces may be 

 invested, also exercise a manifest influence on their emissive 

 power : finally, experiment, correcting the vague conjectures 

 to which the most enlighted minds so foolishly abandon them- 

 selves, shews, that the caloriHc rays proceeding from the sur- 

 face of a heated body, have not the same force or the same in- 

 tensity in all directions ; that the maximum corresponds to the 

 perpendicular emission, and the minimum to the emissions 

 parallel to the surface. 



Between these two extreme positions, how is the diminu- 

 tion of the emitting power effected ? Leslie was the first to 

 attempt the solution of this important question. His observa- 

 tions seemed to prove that the intensities of the emitted rays 

 are proportional — I must, gentlemen, make use of the scientific 

 expression — to the sines of the angles which these rays form 

 with the heated surface ; but the quantities on which it was ne- 

 cessary to operate were too small, and the uncertainty of the 

 thermometric determinations, compared to the whole effect, 

 was, on the contrary, too great, not to cause extreme distrust. 

 Well, gentlemen, a problem which had defied all the processes 

 and all the instruments of. modern physics, was completely 

 solved by Fourier, without attempting any new experiment. The 

 required law for the emission of caloric was, with a sagacity 

 which cannot be sufficiently admired, discovered in the most 



