On lines174-77, Stuart Peterfreund states: "In its cumulative effects, the speaker argues, loves, purifies the desiring subject with something approaching an idealized alchemical efficacy, relieving that subject of a burden that Blake might call selfhood. [...]. Approaching an idealized alchemical efficacy, but neither attaining nor sustaining it: one can approach the boundary separating the material and the immaterial, but she or he cannot cross that boundary in this life, nor can she or he remain at that boundary for long" (281).
But it may be an illusion on the part of both the poet and reader to think that there is an inner-outer distinction anyway. The "veil" (376) can be thought of as a boundary as well, a female-specific object that can serve as a signifier for the Derridean hymen, which Derrida discusses in Dissemination.
Peterfreund, Stuart. Shelley Among Others : The Play of the Intertext and the Idea of Language. Baltimore: Johns Hopkins UP, 2002.