.
The question now arises, with what velocity will the waves be propagated in a medium thus constituted of an immoveable and a moveable part, when for the sake of simplicity we suppose the body to be moving in the direction of the propagation of the waves?
Fresnel considers that the velocity with which the waves are propagated then becomes increased by the velocity of the centre of gravity of the stationary and moving portions of æther. Now
being the velocity of the body,
will be the velocity of the centre of gravity of the system in question, and according to the last formula this expression is equal to
Such, then, is the quantity by which the velocity of light will be augmented; and since
