Every human has a blind spot in each of their eyes. Position your face between the 
X and the 
I below focusing on the 
dot between them. Close your right eye and look across your face at the letter 
I with your left eye. Now move your face back and forth at the screen, at a certain distance the 
X should vanish somewhere between 30cm away.
 
 
_______________X_________________.______ ____________I
 
This happens to all animals except squid, so if it doesn't work for you then the only logical conclusion is that you are a squid.