Ted Underwood does not believe that Shelley's empowering of Emily does not mean that he is challenging contemporary gender roles:
Like Apollo, Emily penetrates, animates, and transfigures both the world and the mind. That Shelley assigns this role indifferently to male and female figures is not especially liberating. Apollo, one should recall, is more a power than a perspective, and Emily acquires his power but not his abstractly first-person voice. "Epispsychidion" does nothing, in short, to rise above the masculanism inherent in the conventional representation of woman as muse. It is nevertheless an important stage in the development of Shelley’s theory of poetry, because it begins to bring his scientific conception of poetry as a transforming light into connection with more familiar tropes of aesthetic theory” (314).
Yet the fact that Emily is figured as a comet brings forth phallic implications. I accept Underwood's assertion that Shelley was influenced aesthetically by advancements in science and technology, but I think Underwood's dismissal of the notion that masculism was being challenged by a reconception of the feminine muse is somewhat problematic. Consider, for instance, the following page, which records Richards Holmes's assertion that the poet's position as a receiver of the comet suggests that Shelley is adopting an androgonous perona.
But what Underwood says about Shelley's scientific influences are useful to our understanding of Shelley's "legislative" theory of poetry--"Poets are the legislators of the world"--as articulated in the Defence of Poetry:
His "Defence of Poetry," for instance, tells us that poetry "transmutes all that it touches, and every form moving within the radiance of its presence is changed by wondrous sympathy to an incarnation of the spirit which it breathes." But for Shelley, the radiance of poetry is an aspect of all perception; never fully present in a written poem, it "is that which comprehends all science and that to which all science must be referred" (293).
Romantics have turned Shelley’s figure of a transmuting light to different ends by using it as a theory of the historical relation between poetic and other discourses. Implying that poems alter the mere knowledge they encounter into a form of being of rare form of power, this theory of literary history excludes scientific ideas from serious critical consideration. Yet Shelley’s figure for poetic transmutation—a radiance that reveals the world only by reproducing the world as an incarnation of its own power—was itself drawn form the science of his time. (302)
Underwood, Ted. "The Science in Shelley's Theory of Poetry." Modern Language Quarterly 58 (1997): 298-321.