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The Standard Method for Finding Your Blind Spot
Use for Your Left Eye
Shut your right eye (block with your hand if that is difficult to to)
You may have to move your head back and forth to find the distance where the yellow spot seems to vanish.
With your left eye, look at the X
Move your head toward the computer screen, always looking directly at the X
Be aware of the yellow spot in the black rectangle, but don't look at it
At some point (the correct distance to the screen varies with screen resolution) the yellow spot will vanish
The yellow spot has vanished because it's image is in the blind spot of your left eye and you cannot see it. Your brain has 'filled in' the blank hole in your vision with the surrounding information, which in this case is the black of the rectangle.
Textbooks invite you to turn your book upside down to repeat the experiment with your right eye. A friend commented to me that that was awkward to do with your computer monitor, so here is the X-and-Spot for the right eye.