572 



In September 1847, MM. Foucault and Fizeau read to the Aca- 

 demy an account of their researches on the interference of the calo- 

 rific rays. By the use of very small spirit thermometers of great 

 delicacy, they were enabled to detect alternations of intensity corre- 

 sponding to those of illumination in the fringes produced by the 

 mirrors of Fresnel. By the same means they detected alternations 

 of temperature, corresponding to and coincident with those of illumi- 

 nation, in the interrupted interference-spectrum obtained by the 

 methods employed in their last-mentioned researches ; but the alterna- 

 tions of temperature were not confined to the region of the visible 

 rays ; they extended into the region of the invisible heat-rays of Sir 

 William Herschel, situated beyond the extreme red. The authors 

 established also the diffraction of heat, having succeeded in showing 

 that the heat at a point a little outside the geometrical shadow of an 

 opaque body with a straight edge, was greater than at such a distance 

 from the shadow that the influence of the body was insensible. 



On the 6th of May, 1850, M. Foucault communicated to the French 

 Academy an account of a highly ingenious and striking experiment 

 relating to the velocity of light*. The reflexion and refraction of 

 light had long before been explained, both on the theory of emissions 

 and on that of undulations. According to both theories, the velocity 

 of light within a refracting medium is different from the velocity in 

 air ; but according to the theory of emissions it is greater, in the 

 ratio of the refractive index to unity, while according to the theory 

 of undulations it is less in the inverse ratio. The progress of opti- 

 cal science had since been such, that the theory of emissions was 

 almost abandoned, while the theory of undulations continually 

 gained fresh support from new phenomena. The effect of the in- 

 terposition of a thin transparent plate in the path of one of two 

 interfering streams, which had been deemed a crucial experiment, 

 was wholly in favour of the theory of undulations. Still, the con- 

 clusion was only an optical inference from the observed result ; the 

 time of transit of the light did not intervene mechanically in the 

 experiment. M. Arago had proposed to employ the revolving mirror 

 of Mr. Wheatstone to decide the question mechanically, in a manner 

 closely resembling that employed by Mr. Wheatstone in measuring 



* Comptes Rendus, torn. xxx. (1850) p. 551, and Aunales de Chimie, torn. xli. 

 p. 129. 



