It is a remnant of the dual grammatical number that was formerly used with all nouns. Nowadays, however, it is only used with parts of body that come in pairs - ears, eyes, hands, legs. Note that this doesn't apply to derivative words like table "legs".
Yes, I realized it had to do with the dual number and that's how the form očima has been formed originally. I just found it unusual that the adjective also had to be modified in a similar way.
dobrý den, snad mohu psát česky :-) přijde mi, že bychom tuto větu do češtiny přeložili spíše jako "na vlastní oči". Je to možná varianta nebo by anglická věta musela být také jiná?