Some time ago I came across a secondary web source on measurement of light speed in water made by Foucault around 1850. I append its redrawn scheme below (light is reflected from the rotating mirror to the fixed one and from the later again to the rotating one which slightly rotates in the meantime so the light ends in the direction labeled air rather than back in the light source).
When Foucault inserted a container filled with water between the mirrors then light was reflected back at larger angle (water), because light is slower in water.
Which velocity is exactly measured in this experiment? Phase velocity (light source is continuous, similar to infinite monochromatic wave), group velocity (which usually applies to a wave packet - is such a packet somehow created by rotation of mirror?) or another one?
Edit (based on the answer by miceterminator):
How would the result (the angle) change in case of negative group velocity (which, as far as I know, is possible)?