Human eyes have a blind spot where the optic nerve exits the back of the eye. (This area has no light receptors because the nerve is in the way.) How do you find it?
Look at the red cross on the left with your right eye, closing your left eye. Move your head slowly closer to the image, then further away. At a certain distance the blue dot on the right will disappear. That's your blind spot!
Your eyes have three types of color receptors: red, green, and blue. After-images appear when you stare at a particular color for too long and those receptors get tired. So stare at the white dot at the center of this image without blinking.
When you suddenly switch to looking at a black-and-white image, those tired receptors won't be working as well, leaving an after-image of the reverse colors. See it?
Most people have a dominant eye. To find out which is yours, make a triangle between your thumbs and forefingers. With both eyes open, look at an object several feet away, centering it inside the triangle.
Close one eye, then the other. The view from the dominant eye will look like the image on the left, and the view from the non-dominant eye will look like the image on the right.