There came into my dream a woman, stuttering,
cross-eyed, stumbling along on her maimed feet,
with ugly yellow skin and hands deformed.
I stared at her. And as the sun revives
a body numbed by the night's cold, just so
my eyes upon her worked to free her tongue
and straighten our all her deformities,
gradually suffusing her wan face
with just the color Love would have desired.
And once her tonge was loosened by my gaze
she started singing, and the way she sang
captured my mind -- it could not free itself.
Purgatory XIX. 7-18
During the night Dante dreams of a Siren. What is fascinating (and frightening) is the way in which she is tranformed before his eyes from repulsive to desirable. Musa (207) points us to Carroll (p.247) and concludes that perhaps what is happening is the gradual change from being repelled by vice to acquiescence, and finally to embrace.
What exaclty is this dream about? It is about Dante's ability to transform the horror of the Siren into something desirable, merely by the force of his own gaze. This is not a tranformation she makes of her own power. The human soul, looking at sin square in the face, should recognize its "ugliness". But then the change begins. Sin does not in itself become less ugly; we make it so. Somewhere in the back of my mind I have a recollection of ancient theories of the power of sight - a sort of projecting of the power of sight outward to the object seen. There may be something of that notion to this episode in Purgatory. But since I'm too slothful to look it up, I'll leave the analogy from ancient medical theory for the time being.
What is significant is that the Siren would not have the power over Dante's mind ("it could not free itself") had he not in some sense given that power to the Siren. It is he who "projects" on her the aspect of desire - "just the color Love would have desired". It is he who gives her this pleasing appearance, and in the end he is trapped by her. He has transformed her by his gaze, and in the end he cannot escape from hers. It is an image of the way in which we gloss over sin, at first perhaps tolerating it, and in the end denying it to be sin at all. We come to think it beautiful. Once we have given it this pretense of good and beauty, we are then powerless to escape it.