CHICAGO — Tropical palms, desert tents and even a pair of stuffed elephants helped transform the cool marble halls of the Field Museum into a jungle paradise for the museum’s centennial ball. The Saharan ambience was appropriate, as the ball also celebrated the opening of “Africa,” a new permanent exhibit. About 850 guests attended, raising $300,000 for the natural history museum. With images from the exhibit projected onto the building’s pillared facade, the event could be seen across the city.