Proverb meaning that the eyes reflect a person's heart or character (used similarly to 'the eyes are the windows to the soul').
面接では言葉だけでなく目の表情も大事だ。目は心の鏡だと思う。
In interviews, not just words but eye expressions matter. I think the eyes are the mirror of the heart.
疲れているときは目に現れるから、目は心の鏡だと昔の人は言ったのだろう。
When you're tired it shows in your eyes, so perhaps that's why people once said the eyes are the mirror of the heart.