Joyce often uses metaphors to convey lofty ideas about language. In Chapter 4, Part 3, the narrator describes language as a "prism":
Did he then love the rhythmic rise and fall of words better than their associations of legend and colour? Or was it that, being as weak of sight as he was shy of mind, he drew less pleasure from the reflection of the glowing sensible world through the prism of a language manycoloured and richly storied than from the contemplation of an inner world of individual emotions mirrored perfectly in a lucid supple periodic prose?
Here, language is a "prism" through which Stephen may convert the dull events of everyday life into high art. A prism is a piece of cut glass (or other transparent material) that has refracting surfaces that separate white light into a spectrum of colors. Because prisms refract light, this metaphor suggests a process of change or alteration.
He seems torn between the "glowing sensible world" and his own "inner world"; different types of language correspond to each realm, and the "prism" represents the multifaceted perspective that an author of the world could provide (as opposed to the contemplative, perfectible prose of the "inner world.")
This passage also contains a few good examples of alliteration in phrases like "rhythmic rise" and "periodic prose." Alliteration makes the prose lyrical and rhythmic; it also reminds the reader of the heights of grace and beauty to which Stephen aspires in his speech and writing.