画龙点睛
There was a painter in ancient China who was especially adept at painting dragons. Many people, from emperors to Buddhist priests, hired him to decorate their residences, temples, and murals with his artwork.
One day, as the painter was finishing up a mural of four dragons, people walking by noticed that he left the eyes of the dragons unpainted. “How come you are leaving the eyes blank?” the people asked.
The painter replied that if he drew the eyes of the dragons in, the dragons would fly away into the sky. The people were incredulous and urged the painter to finish his mural, so the painter sighed and drew in the eyes of two of the four dragons.
As soon as he finished with the eyes of the second dragon, the clouds in the sky darkened and the winds began gusting. Lightning streaked across the sky and at the clap of thunder that followed, the people saw the two dragons separate themselves from the mural, and fly up into the sky.
画龙点睛 later became a chengyu that indicated that extra touch, that special something, when used at a key moment in a process, (usually storytelling or essay-writing), that makes the work vivid and life-like for the audience or reader.
Below are some other chengyus associated with eyes, eyesight, and vision:
一见钟情 (yi2jian4zhong1qing2): love at first sight
目清眉秀(mu4qing1mei2xiu4): good looking
目迷五色 (mu4mi2wu3se4): confused and dazed by sensual overload
光耀夺目 (guang1yao4duo2mei2) : so bright that it dazzles the eye – dazzlingly beautiful
万目睽睽 (wan4mu4kui2kui2): all eyes on it.
死不瞑目 (si3bu4ming2mu4): die unjustly (“refuse to close eyes in death”)
虎视眈眈 (hu3shi4dan1dan1): covetous stare.
刮目相看 (gua1mu4xiang1kan4): to gaze at with admiration.
You may also want to take a look at : Eye of the Beholder.