It turns out that when they replay a lot of calls linesman are more likely to be wrong on balls ruled out. They did this research at Wimbledon last year and I would not even attempt to go into the science of the eye following a tennis ball. But it means that players should concentrate their challenges on balls that were called out because on in calls the players eyes are as fooled as the linesman. This is interesting and I am sure the players will pick up on it.