Light and Focus

Teaching a friend how to scout and look out for enemy targets last week. I couldn't and still can't remember for the life of me the name of the reason why we look off center in a night situation.  What I mean is this, if you believe your target is at say 12 o'clock, you shouldn't look exactly at 12 o'clock to see your target's movements but instead slightly off otherwise they will appear invisible to you.