Snake-like Reflection¹ of a Bow in the Cup

Nearly 2,000 years ago, during the Han Dynasty, there was a county² magistrate³ called Ying Chen. One summer day, he invited his secretary Du Xuan to his house and treated him with wine. On the north wall of the room hung a red bow. It was reflected in Du Xuan's cup. Du Xuan took the reflection for a squirming snake. He was very frightened but he dared not turn down Ying Chen's offer because Ying was his superior⁴. He had to swallow the wine with his eyes closed.

　　When he was back at home he felt so painful in his chest and stomach that he could hardly eat and drink any more. He sent for the doctor and took some medicine, but nothing could cure him. When Ying Chen asked Du Xuan how he got so seriously ill, Du told him he drank the wine with a snake in his cup the other day. Ying Chen found something strange about that. He returned home , thought hard, but he could not find an answer. Suddenly the shadow thrown by the bow on the wall caught his eye. "That's it!" he shouted. He immediately sent his man to fetch Du Xuan. He seated him where he sat before and offered him a cup of wine. Du Xuan saw the snake-like shadow again. Before Du was scared out of his wits⁵ again, Ying Chen said , pointing at the shadow, "The 'snake' in the cup is nothing but a reflection of the bow on the wall!" Now that Do Xuan knew what it was, he felt much easier. And strange enough, his illness disappeared the next moment.

　　This story was later contracted into the idiom "Bei Gong She Ying'. It is used to describe someone who is very suspicious⁶. It is like saying someone is afraid of his own shadow.

“杯弓蛇影”这则成语的意思是误把映入酒杯中的弓影当成蛇。比喻因错觉而疑神疑鬼，自已惊扰自已。
这个成语来源于东汉.应劭《风俗通义》，时北壁上有悬赤弩，照于杯，形如蛇。宣畏恶之，然不敢不饮。
有一年夏天，县令应郴请主簿（办理文书事务的官员）杜宣来饮酒。酒席设在厅堂里，北墙上悬挂着一张红色的弓。由于光线折射，酒杯中映入了弓的影子。杜宣看了，以为是一条蛇在酒杯中蠕动，顿时冷汗涔涔。但县令是他的上司，又是特地请他来饮酒的，不敢不饮，所以硬着头皮喝了几口。仆人再斟时，他借故推却，起身告辞走了。
回到家里，杜宣越来越疑心刚才饮下的是有蛇的酒，又感到随酒入口的蛇在肚中蠕动，觉得胸腹部疼痛异常，难以忍受，吃饭、喝水都非常困难。
家里人赶紧请大夫来诊治。但他服了许多药，病情还是不见好转。
过了几天，应郴有事到杜宣家中，问他怎么会闹病的，杜宣便讲了那天饮酒时酒杯中有蛇的事。应郴安慰他几句，就回家了。他坐在厅堂里反复回忆和思考，弄不明白杜宣酒杯里怎么会有蛇的。
突然，北墙上的那张红色的弓引起了他的注意。他立即坐在那天杜宣坐的位置上，取来一杯酒，也放在原来的位置上。结果发现，酒杯中有弓的影子，不细细观看，确实像是一条蛇在蠕动。
应郴马上命人用马车把杜宣接来，让他坐在原位上，叫他仔细观看酒杯里的影子，并说：“你说的杯中的蛇，不过是墙上那张弓的倒影罢了，没有其他什么怪东西。现在你可以放心了！”
杜宣弄清原委后，疑虑立即消失，病也很快痊愈了。