Sit at arms length from the screen, close your right eye then stare at the right(blue) dot in Fig. 1 below.
Without moving your gaze from the right dot, concentrate on the left(red) dot then move slowly towards the screen.
At approximately 18 inches the left(red) dot will disappear from view. Continue to move forwards towards the screen
and the left(red) dot will eventually re-appear.
You can repeat his procedure using right eye and by staring at the left(red) dot.
Why do the dots disappear?
Each eye has a blind spot where
the optic nerve meets the back of the eyeball. These blind spots have no light receptors and each is offset
from the centre of the eyeball. The offset of the left eye is to the right and vice versa for the
right eye. Thus the blind spots of each eye do not overlap and the area of each blind spot is compensated for by
light receptors in the other eye.
More on Blind Spots
Sit at arms length from the screen, close your right eye then stare at the blue dot in Fig. 2 below.
Without moving your gaze from the blue dot, concentrate on the black line then move slowly towards the screen.
At approximately 18 inches the line will first appear to distort and bulge outward and then as you move closer a large portion of the
line will disappear from view. Continue to move forwards towards the screen
and the line will eventually re-appear. It is recommended that you tilt your head very slightly to the right to acheive the best results.
This is due to the fact that the blind spot is not only displaced to the side, but is also displaced slightly from the horizontal.
Why does the line distort and bulge?
The point where the blind spot
is located and where the optic nerve meets the back of the eyeball forms a slight indentation in the back of the eye.
Light receptors exist in the the periphery of this "dish" shape indentation and therefor cause the image
in that area to be distorted. The complete disappearance of an image at the centre of the blind spot, as described before,
is due to the complete absence of light receptors.
Seeing is Believing, Isn't It?
Go ahead, count the black dots at the intersection of the white paths, if you can.
No explanation this time.
The dots are obviously just a construct of the eye or brain
or a combination of the two together.
When is a trunk a trunk or even a leg? How many legs does this elephant have?.
Again no explanation.
Is this the example for the new UK Rail Network following the replacement of Railtrack?
Well, are the horizontal lines parallel?
It's your choice!
Rapa Nui or Baffin Island?
One of the monuments on Easter Island or a resident of northern Canada?
Whatever, the choice is in the eye of the beholder.