核心问题
（a）Do you like either of these?
(b) On either side of the river lie corn-fields.
就我掌握的英语知识而言, either 是: “两者之中任一” (one or other of the two) 的意思。我的知识似乎与句(b)相矛盾。
词典解答
either 的第一个词义是指“两者之中任一”：
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Is either of the boys ready? (LDCE) 两个孩子之中有谁准备好了吗?
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You may go by either road. (Palmer) 你可以走两条路之中的任一条。
但是, 当 either 与 side, end, hand, bank 连用时, 它的含义则是“两者之中每一”(each of the two):
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He sat in the car with a policeman on either side of him. (LDCE) 他坐在汽车里，两边各有一名警察。
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There was an armchair at either end of the long table. (ALD) 在长桌的两端各有一把扶手椅。