The mirror gimmick consists of two components, one being a small mirror and a bigger mirror.
In Sonic Adventure, and its enhanced port Sonic Adventure DX: Director's Cut, the mirror gimmick appears exclusively in Sonic the Hedgehog's version of Lost World. When entering a certain room of Lost World, the room itself will be very dark, making it difficult to proceed. The player can light up these rooms with the small mirror, which has light reflecting off its surface. Doing so will cause the player to switch to a first person perspective. While in the first person perspective, the player cannot leave the mirror unless they aim the light beam at the other mirror, which will in turn light up a small portion of the room.