On the effect of the motion of a body upon the velocity with which it is traversed by light

by H. Fizeau

' the velocity of light in water when at rest,

the velocity of the water supposed to be moving in a direction parallel to that of the light. It follows that

is the velocity of light when the ray and the water move in the same direction, and

when they move in opposite directions.

If

be the required retardation and