Stephen says bitterly, "It is a symbol of Irish art. The cracked lookingglass of a servant." M. H. Abrams showed in a classic study that mirrors have long been regarded as symbols of art conceived classically as a faithful mimetic representation of external reality. Stephen's characterization of the mirror seems to suggest that faithful representation of reality is difficult within the confines of colonial subjection.
When he was living outside of Ireland, and struggling to get Dubliners into print, Joyce wrote to the publisher Grant Richards, “It is not my fault that the odour of ashpits and old weeds and offal hangs round my stories. I seriously believe that you will retard the course of civilization in Ireland by preventing the Irish people from having one good look at themselves in my nicely polished looking-glass” (Letters, 63-64). His book of stories, then, intends to show Caliban his face in a mirror. Seeing themselves represented unflatteringly may liberate the Irish from ignorance and complacency. Stephen has not yet managed to write anything like the searing but honest portrayal of external reality described in Joyce's letter to Richards, and in Telemachus the despondency that he feels being back in Dublin appears to include doubt that he can represent Ireland truly while living trapped within it. At the end of Aeolus, however, he resolves to attempt it.
In James Joyce's Ulysses: A Study (1930), Stuart Gilbert noted an additional context for the image of a “cracked looking glass” that may complicate or even refute the foregoing reading. In The Decay of Lying (1889), Oscar Wilde uses the phrase to object to the notion that art mirrors external reality. Wilde espoused the Romantic belief in individual genius—the view that Abrams characterizes as a "lamp." In this view, the genius of the artist is more valuable than what most human beings regard as reality. The fact that Stephen repeats Wilde's phrase verbatim, and the fact that Mulligan has just invoked another work by the same author, may recommend this anti-mimetic reading of the sentence.