By means of this spell, the wizard changes a normal mirror into a scrying device similar to a crystal ball. The details of the use of such a scrying device are found in the DMG (in Appendix 3: Magical Item Descriptions, under the description for the crystal ball).
The mirror used must be of finely wrought and highly polished silver and cost not less than 1,000 gp. This mirror is not harmed by casting the spell, but the other material components—the eye of a hawk, an eagle, or even a roc, and nitric acid, copper, and zinc—are used up.
The following spells can be cast through a magic mirror: comprehend languages, read magic, tongues, and infravision. The following spells have a 5% chance per level of the caster of operating correctly: detect magic, detect good or evil, and message. The base chances for the subject to detect any crystal ball-like spell are listed in the DMG (again, in Appendix 3: Magical Item Descriptions, under the description for the crystal ball.