This is a great illustration of what people were taught about scanning outside the cockpit when they went through flight training. They were told to scan the horizon for a short distance, stop momentarily, and repeat the process. They were told why this was the most effective technique to locate other aircraft. It was emphasized repeatedly to not fix their gaze for more than a couple of seconds on any single object. The instructors, some of whom were veterans with years of experience, instructed them to continually “keep your eyes moving and your head on a swivel” because this was the best way to survive, not only in combat, but from peacetime hazards (like a midair collision) as well.
They basically had to take the advice on faith (until they could experience for themselves) because the technology to demonstrate it didn’t exist at that time. Click on the link below for a demonstration