The looking glass, also known as the double monocle, is an ancient item, given to Cass by the fortune teller when she is in her chocolate coma state. It is also worn by Benjamin Blake, which Max-Ernest steals from him. It is an ancient Midnight Sun relic, supposedly dating from Lord Pharaoh's time.Edit
The eye piece can let you see things impossible with the naked eye. However, it is different for all people. Max-Ernest saw himself in the future, Benjamin saw what people were thinking, and Cass mentally saw things far away or hidden behind other objects. This object is the symbol for sight in the series. As Max-Ernest points out, it's a visual visual pun, as mono means one, but it has two lenses. It's visual twice, because you can see the pun, but it's also meant to be seen through.