One of the many symbols in this poem is the "spot / Of joy [in] the Duchess’s cheek." This "spot of joy" symbolizes, at least from the Duke's perspective, the Duchess's sexuality. Indeed, in the Victorian period when this poem was written, blushing in women was interpreted as an expression of sexuality. This blush in the Duchess is obviously something which infuriated the Duke, probably because he took it as a sign that the Duchess was unfaithful. He mentions it twice more in the next few lines.
Later in the poem, the Duke is bemoaning the fact that the Duchess didn't value his "favor" as much as he thought she should have. He says that she valued it no more than a number of other, trivial things, such as "The dropping of the daylight in the West." The symbol of the setting sun in this quotation ominously foreshadows the darkness that was to come in the Duchess's life (most obviously her death).
The Duke also complains that the Duchess valued his "favor" no more than "The bough of...
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