He found that the velocity of light in a liguid is smaller than that in vacuum, depending on how dense that liguid is.
Light travels in a motionless liquid with a particular velocity w .
In accordance with the principle of relativity we shall certainly have to take for granted that the propagation of light always takes place with the same velocity w with respect to the liquid, whether the latter is in motion with reference to other bodies or not. The velocity of light relative to the liquid and the velocity of the latter relative to the tube are thus known, and we require the velocity of light relative to the tube. Here, W signifies the velocity relative to the tube.