阅读下面这首宋词，然后回答问题。
蝶恋花^①
陆　游
桐叶晨飘蛩夜语。旅思秋光，黯黯长安路。忽记横戈盘马处，散关清渭应如故。江海轻舟今已具。一卷兵书，叹息无人付。早信此生终不遇，当年悔草《长杨赋》^②。
[注] ①本词是陆游晚年被罢官后所作。②《长杨赋》：汉代扬雄为讽谏汉成帝游幸长杨宫而作。
首句景物描写有何作用？(3分)
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下片表达了作者怎样的情感？请作简要分析。(4分)
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