Mirrors were divinatory instruments long before they were furniture. Catoptromancy, scrying by mirror, runs from the ancient world through Renaissance magic, and folk practice kept mirror divination alive at the domestic level, the girl who looks in the glass by candlelight to see a future husband. The superstitions travelled with the practice: the broken mirror's seven years of ill luck, and the custom of covering mirrors in a house of mourning, kept the glass charged as an object where seeing and fate touch. A dream mirror inherits all of it.
The old handbooks read the reflection by its fidelity, a true likeness one way, a distorted, aged or absent one another, and that instinct persists in the modern reported forms: the reflection that does not match, that moves independently, that shows another face or none. Interpreters across the psychological schools read these toward self image and its shocks, the gap between the self as lived and the self as seen.
The Jungian tradition gives the image its strongest modern frame, the mirror as the first test on the way into the unconscious: what looks back at the threshold is the shadow, everything the dreamer is and prefers not to see. Whoever will not face the reflection, that school holds, gets no further. The reading is Jungian doctrine, recorded here as such, and the older folk registers run happily on beside it.