It is critically important to understand that the reasons behind this optical illusion are also the reasons why you shouldn't stare down the sights forever and an age before letting a shot off.
|Stare at the three coloured dots on the woman's nose for 30 seconds, then look at a brightly lit white piece of paper or wall and blink. You should see a photo-positive image.|
The answer to the riddle of why we see a positive image of a woman, having stared at the negative image lies in the chemical magic of how our eyes work. When light shines on certain chemicals in the cells of the retina, a reaction is triggered which results in nerve impulses being fired. Our brain interprets these nerve impulses and we see an image as a result. These photosensitive chemicals are then regenerated for further use.
The problem is that there are finite quantities of these chemicals in our eyes and the chemical process which recycles them takes a little time. Normally this doesn't matter because your eyes are constantly moving and the patterns of light falling on your eyes change also, so the regeneration process can keep up; however if you're very still and looking at the same image for a long time, the supply of photosensitive chemicals is exhausted in the retinal cells. When you look at the blank piece of paper, the overstimulated retinal cells don't react in the same way whereas un-stimulated cells do, producing the same pattern of reactions that you would get if you were looking at a positive image. The image has been "burned" into your retina and you are no longer seeing what is actually there.
Just as in the Facebook post where you see a ghostly afterimage of a woman, when looking through the sights of your rifle the image of foresight and target becomes distorted. Holding on to shots for too long before letting them off is not a great recipe for phenomenal accuracy. Let the rifle settle naturally on target, breathe in, breathe out, acquire the sight picture and then fire the shot within 2 - 8 seconds.