One point I would like to add is that staring at a target will burn the image into your retina! It was explained to me by Mike Taylor a complete Guru for shooting and eyes.
Anyway, your brain will stop updating information as it gets used to an image not changing...so for us target people looking at a black circle, the longer you look the less lightly what your looking at is the actual target, shooting an 8 is no problem and experiments on scatt show the shot was good but the result was not at all where you thought you were aiming. So looking at a target for no more than four seconds is what is generally accepted then it's time to build the shot again...
I used to use Clarymist (tescos i think), you spray it onto your closed eyes and it is a synthetic tear formular! Good tip for Mr Coolings wife Di!