Title: Vacuum.
Question: Ibn al-Haytham used geometry to demonstrate what?

Title: Slavs.
Question: The Indo-Europeans who remained after the migrations became speakers of what?

Title: Antenna (radio).
Question: What would the antenna you are using take advantage of to allow it to work in multiple locations?

Title: Palermo.
Question:
Who was blamed for the week long rebellion of 1866?