(Image from the Internet)
Ying Chen invited Du Xuan to drink, and the moment Du Xuan lifted the wine glass, there was a snake-like thing in the cup that he also drank. Du Xuan was confused and did not dare to drink again, and fell ill after returning home.
After Ying Chen knew, he was very guilty. Sitting in Du Xuan's position at that time, pouring wine and looking closely, it turned out that the colored bow hanging on the wall was reflected in the cup, and it shook like a snake with the wine. Ying Chen deliberately explained this to Du Xuan, invited him again to sit in his original seat, poured wine for him to see, and became ill.
The bow snake shadow in the cup means to treat the bow shadow in the wine glass as a snake, and to disturb yourself because of doubt. The metaphor is a vain panic about things that don't exist.