Anything which limits our peripheral vision, will eliminate the initial motion cues, which cause us to shift our point of view. In flying, such things as the wings, other parts of the fuselage, our instrument panels, and our clothing can interfere with this process. It only takes a limitation to the peripheral vision on one eye to completely hide an aircraft as big as a jetliner that is perhaps only 2 kilometres away. Smaller aircraft such as gliders can easily slip from notice even closer in.
Given the narrow area in which our eyes can focus, we can see that in the flying environment it is very easy to miss objects that are blocked from our view by things like canopy frames, distortions in the canopy, or even dirt. For example with gliders, consider a piece of tape that is 5/8” wide (15 mm), used to fix the yaw string to the canopy. Another glider approaching head-on can be completely obscured with this tape at a distance of less than ¾ a nautical mile away (1330 metres). Even as close as 500 feet (150 m), the fuselage may be still be hidden. If we consider the first case, with both aircraft flying at 55 knots (100 km/hr), we have 33 seconds to discover the aircraft, and avoid collision. When we are 500 feet apart we have less than 3 seconds to react.
This means that either we have to limit the obstructions to our view, or compensate for them with movements of our eyes and head. We must really resist the urge to fixate on any one point for and extended period of time. This does not mean that we must move like a rabbit, but we should change our viewpoint every 3 seconds or so particulary if we have some obstructions to our vision.