What is the poetic eye of On Passing Jia Yi's Family in Changsha (1)? Please explain the reason with the whole poem.

(1) The word "sadness" runs through the end of the poem, laying the melancholy tone of the whole poem, which not only fits Jia Yi's life, but also implies the tragic fate of Liu Changqing's own exile.

(2) The necklace is vaguely related to himself from Jia Yi's ignorance. A word "Jude", known as "Youdao", was so kind to Jia Yi, what's more, it was inevitable that Tang Daizong, who was groggy and incompetent at that time, belittled Liu Changqing again and again. The strokes of innuendo point to the present emperor in a zigzag way. Xiangshui is ruthless and has been flowing for many years. How could Qu Yuan of Chu know that hundreds of years later, Jia Yi would come to the shore of Xiangshui to mourn himself; Jia Yi in the Western Han Dynasty did not expect that Liu Changqing would meet the bleak autumn wind to mourn his site after nearly a thousand years. The heart of the latecomers should not hate the ancients for coming down to the ground to listen, and no one in the world can understand. The poet is sincerely looking for a bosom friend, and his mood of depression and negative behavior is very emotional and real.