The path of a ray of light in irrotationally moving media was first investigated mathematically by Fresnel, who showed that to account for the observed phenomena it is necessary to suppose that, the æther being fixed, the medium imparts of the amount of its own motion, resolved in the line of the refracted ray, to the advancing disturbance. This conclusion is also a necessary consequence of the modern Theory of Relativity, which, while holding that, with reference to axes with regard to which the medium is stationary at any point, the speed of light depends only on the refractive index, for any other system leads to the above result.