Airy, in 1871 filled up a telescope with water—but failed to detect any change in the aberration. Thus we get both in the case of Arago prism experiment and Airy-Boscovitch water-telescope experiment, the very startling result that optical effects in a moving medium seem to be quite independent of the velocity of the medium with respect to Fresnel’s stationary ether. _Fresnel’s convection coefficient k = 1 - 1/μ^2._—Possibly some form of compensation is taking place. Working on this hypothesis, Fresnel offered his famous ether convection theory. According to Fresnel, the presence of matter implies a definite condensation of ether within the region occupied by matter. This “condensed” or excess portion of ether is supposed to be carried away with its own piece of moving matter. It should be observed that only the “excess” portion is carried away, while the rest remains as stagnant as ever. A complete convection of the “excess” ether ρ′ with the full velocity _u_ is optically equivalent to a partial convection of the total ether ρ, with only a fraction of the velocity _k_. _u_. Fresnel showed that if this convection coefficient _k_ is 1 - 1/μ^2 (μ being the refractive index of the prism), then the velocity of light after refraction within the moving prism would be altered to just such extent as would make the refractive index of the moving prism quite independent of its “absolute” velocity _u_. The non-dependence of aberration on the “absolute” velocity _u_, is also very easily explained with the help of this Fresnelian convection-coefficient _k_. _Stokes’ viscous ether._—It should be remembered, however, that Fresnel’s stationary ether is absolutely fixed and is not at all disturbed by the motion of matter through it. In this respect Fresnelian ether cannot be said to behave in any respectable physical fashion, and this led Stokes, in 1845-46, to construct a more material type of medium. Stokes assumed that viscous motion ensues near the surface of separation of ether and moving matter, while at sufficiently distant regions the ether remains wholly undisturbed. He showed how such a viscous ether would explain aberration if all motion in it were differentially irrotational. But in order to explain the null Arago effect, Stokes was compelled to assume the convection hypothesis of Fresnel with an identical numerical value for _k_, namely 1 - 1/μ^2. Thus the prestige of the Fresnelian convection-coefficient was enhanced, if anything, by the theoretical investigations of Stokes. _Fizeau’s experiment._—Soon after, in 1851, it received direct experimental confirmation in a brilliant piece of work by Fizeau.