412 JOSEPH FOURIER. 



the thickness of the outer coating of these same surfaces, 

 exercise also a manifest influence upon their emissive 

 power. Experience, finally, rectifying the vague predic- 

 tions to which the most enlightened minds abandon them- 

 selves with so little reserve, shows that the calorific rays 

 which emanate from the plane surface of a heated body 

 have not the same force, the same intensity in all direc- 

 tions ; that the maximum corresponds to the perpendicu- 

 lar emission, and the minimum to the emissions parallel 

 to the surface. 



Between these two extreme positions, how does the 

 diminution of the emissive power operate ? Leslie first 

 sought the solution of this important question. His ob- 

 servations seem to show that the intensities of the radi- 

 ating rays are proportional (it is necessary, Gentlemen, 

 that I employ the scientific expression) to the sines of the 

 angles which these rays form with the heated surface. 

 But the quantities upon which the experimenter had to 

 operate were too feeble ; the uncertainties of the ther- 

 mometric estimations compared with the total effect were, 

 on the contrary, too great not to inspire a strong degree 

 of distrust : well, Gentlemen, a problem before which all 

 the processes, all the instruments of modern physics have 

 remained powerless, Fourier has completely solved with- 

 out the necessity of having recourse to any new experi- 

 ment. He has traced the law of the emission of caloric 

 sought for, with a perspicuity which one cannot suffi- 

 ciently admire, in the most ordinary phenomena of tem- 

 perature, in the phenomena which at first sight appeared 

 to be entirely independent of it. 



Such is the privilege of genius ; it perceives, it seizes 

 relations where vulgar eyes see only isolated facts. 



Nobody doubts, and besides experiment has confirmed 



