The Looking Back Glass is a magic mirror that shows those who use it events in the past when they happened just as they happened. The users can also enter the mirror to get a close up look. There is no danger of changing history this way as the users can't be seen or heard.
Calista received a Looking Back Glass from her mother for her birthday. In "Through the Looking Back Glass", she and Sofia use the Looking Back Glass to find out about the incident that broke the relationship between Cordelia the Conjuror and her little brother Cedric. During the use, Sofia learns that King Roland I originally was going to pass down his crown to his daughter Tilly, but instead gave it to his son Roland II.
The incident that broke the relationship is revealed to be when Cedric created magic smoke that ruined his sister's hair and his attempt to fix it only turned it green. A second use reveals this was actually Cordelia's fault: She knocked her hair potion into the serum and turned it white, which is why it changed her hair. Sofia and Calista then show this moment to Cordelia and Cedric.