]" Is it refering to the ability of looking at the diagram, No, it is referring to your ability to comprehend the diagram.
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soheil1When a (chess) book writesNo, it is referring to your ability to comprehend the diagram.
"As you can see in diagram 108, [..]" Is it refering to the ability of looking at the diagram,
soheil1How?Why not ability of seeing the diagram?Use your common sense: there is no point in the author wondering whether the reader is blind or not.