158 JOSEPH FOURIER. 



aid of this aitifiee to arrest the heat which would have burned their 

 eyes. 



In the experimental sciences, the epochs of the most brilliant progress 

 are almost always separated hy long intervals of almost absolute repose. 

 Thus, after Mariotte, there elai)sed more than a centnry without history 

 having to record any new property of radiating heat. Then, in close 

 succession, we find in the solar light obscure calorific rays, the existence 

 of which could admit of being established only with the thermometer, 

 and which may be completely separated from luminous rays by the aid 

 of the prism ; we discover, by the aid of terrestrial bodies, that the 

 emission of caloric rays, and consequently the cooling of those bodies, 

 is considerably retarded by the polish of the surfaces ; that the color, 

 the nature, and the thickness of the outer coating of these same sur- 

 faces exercise also a manifest influence upon their emissive power. 

 Experience, finally, rectifying the vague predictions to which the most 

 enlightened minds abandon themselves 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 directions; that 

 the maximum corresponds to the perpendicular emission, and the min- 

 imum 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 import- 

 ant question. His observations seem to show that the intensities of 

 the radiating rays are jiroportional (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 

 thermomctric estimations compared with the total effect were, on the 

 contrary, too great not to inspire a strong degree of distrust ; well, gen- 

 tlemen, a iiroblem before which all the processes, all the instruments of 

 modern physics, have remained jiowerless, Fourier has completely solved 

 without the necessity of having recourse to any new experiment. He 

 has traced the law of the emission of caloric sought for, with a perspi- 

 cuity which one cannot sufficiently admire, in the most ordinary pheno- 

 mena of temperature, 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 the fact, that 

 in all the points of a space terminated by any envelope maintained at a 

 constant temperature, we ought also to experience a constant tempera- 

 ture, and precisely that of the envelope. Now, Fourier has established 

 that if the calorific rays emitted were equally intense in all directions, 

 if the intensity did not vary proportionally to the sine of the angle of 

 emission, the temperature of a body situated in the inclosure would 

 depend on the place which it would occupy there ; that the temperature 



