the bitumen hardened in the brightly lit areas, but in the dimly lit areas it remained soluble and could be washed away with a mixture of oil of lavender and white petroleum. a very long exposure in the camera was required. sunlight strikes the buildings on opposite sides, suggesting an exposure that lasted about eight hours, which has become the traditional estimate. a researcher who studied niépce's notes and recreated his processes found that the exposure must have continued for several days.