It seems to be strange that there is a blind spot in the eye. Here is an interesting experiment   that can make something _ when one eye is open. Make a card about the size of a postcard and write two English letters "L" and "R" on it. "L" is on the left and "R" on the right. First, hold the card about 80 cm away and you can see both the letters. Then close your right eye and look at the letter "R" only with your left eye. And now, as you move the card slowly towards you, you'll find the letter "L" disappearing. But if you move the card nearer to your face, the letter will be seen again. Now do the same experiment with your left eye closed; you'll find the letter "R" disappearing. Why do the letters disappear? It is because there is a blind spot in the eye. When the image   of the letter falls on the blind spot, it won't be seen. That's why one of the letters disappears. The writer of the passage thinks that   _   there is a blind spot in the eye.
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