Walk up to the sarcophagus lying inside a glass cabined. On the ground, to the left of the glass cabinet, you can move the carpet. Once the carpet is out of the way, you will be able to see a trap door. Open the trap door. Use the lever from the tile puzzle on the machine inside. When the lever is in place, pull it to reveal a hidden entrance in one of the other rooms. Before leaving however, combine the lamp and the oil flask in your inventory. Light your lamp by using it on the gas lamp hanging over the glass cabinet with the sarcophagus.