Thanks clmax, another piece of trivia for me to stow away in my head.
Before the IRVR machines were installed at my base (UK, not France), the RVR was observed visually. The fire crews were deployed to the side of the runway and counted the visible runway edge lights. This was reported to the tower, mutliplied by the distance between the lights, and passed on to the pilots. All legal in UK, if somewhat old fashioned.