Once inside the museum, ignore some of the new objects and proceed to the assistant right away (screen). Open your inventory and choose the list of artefacts received from Herodotus. Show the list to the assistant by clicking on him. You will be rewarded with an interesting cut-scene. Someone tries to kill Van Helsing, but thankfully our main hero manages to survive. The assailant fleds the museum before you're able to catch up to him.
Before you decide to talk to the assistant again, you should consider taking two new objects. Start off by pulling a Knife from the wall. After that, take a Broken piece of mirror. You may talk to the assistant now, however make sure that the list of artefacts is your active item. You will be allowed to take the artefacts you need in exchange for finding some of the stolen goods. This means you will have to find the thief in the first place. Read a new report in your diary - The Necklace. You should also open your inventory, because two items have been added to your list - a Receptacle, the capacity of a hinu and a Necklace. Exit the museum.