Although commentators have recognized that the painting alluded to in William Carlos Williams' "Portrait of a Lady" is by Fragonard and not by Watteau, as far as I can determine no one has noted the particular nature and function of the overlapping references to these painters, nor, it seems, has anyone made an exact attribution of the poem's alternating voices. Both these clarifications are essential for interpreting the poem.
The poem presents a dialogue between a man and a woman or an imaginary dialogue within a man's mind reflecting the likely reactions of a woman to his elaborate and somewhat artificial but nevertheless delicate, tender, and mellifluous praise of her loveliness and sexual appeal. Jean Antoine Watteau (1684-1721) and Jean Honore Fragonard (1732-1806) were both French painters of the baroque or rococo style, much of whose work presents aristocratic people in elaborate poses. The painting which portrays a lady's slipper suspended in the air in front of the lady on a swing is incorrectly attributed to Watteau by one of Williams' speakers. The painting is "La Balancoire" ("The Swing") by Fragonard. The reference to Fragonard must, then, be a correction of the statement about Watteau. This makes it very likely that the sentence "The sky / where Watteau hung a lady's / slipper" is spoken not by the woman but by the man and is his answer to the question "Which sky?" The sentence, then, represents a complication of the interacting voices, for the man is capable of satirizing his own viewpoint, or at least of placing it in perspective as rather mannered. If the sentence about Watteau's supposed painting were spoken by the woman, it would almost certainly take a question mark as an implied continuation of "Which sky?"
After speaking this phrase, the man proceeds to call her knees a southern breeze. The immediately following "—or a gust of snow" is her deflating continuation of his description and a playful rebuff of the direction of his sexual advance. The question about Fragonardis asked by the woman as a correction of his remark about Watteau. Surely ''as if that answered anything" cannot be spoken by the same voice. Rather it is the man's acceptance of her factual correction and also an insistence that the mentality or artistry of Fragonard is not relevant to his own sincerity or accuracy, or even to his own right to an elaborate mode of expression. "Ah, yes" represents the man's attempt to recover his composure and his line of thought, and he proceeds to incorporate the woman's cooling of the description by sardonically accepting the fact that attention moves below rather than above the knees, though he recovers the note of praise by assigning delicate summer loveliness to the portion of her body below the knees. With "the sand clings to my lips—" the man accepts a tentative and self-mocking defeat, the sand representing her success at warding off his incipient physical gesture, and the "Agh, petals maybe" shows him trying to recover his stance by suggesting that the shore is made of fallen petals rather than of rebuffing sand. But with the woman's insistence on knowing "Which shore?" his pride in the genuineness of his expression and feeling surges up and he attempts to retrieve his position through assertion that by being made of petals the elevated world of her body does indeed defy the world of logic. This interpretation assigns passages with the exclamation "Agh!" to alternate speakers (though it has the exclamation mark only with the first occurrence), but awareness of how the man almost shares and partly conducts the woman's deflation of him should justify Williams' use of an identical expression of feigned disgust by both speakers.
From "Dialogue and Allusion in William Carlos Williams' 'Portrait of a Lady.'" Concerning Poetry 10:2 (Fall 1997).