Here are some interesting facts about Fresnel Lenses:

The Second Order Fresnel Lens from the White Shoal, MI lighthouse, now located in the Great Lakes Shipreck Museum at Whitefish Point, is:

9 feet in diameter
has 344 leaded crystal prisims
weighs 3,500 pounds
was visible for 28 miles
had to be rewound every two hours an eighteen minutes.

Most people don't realize just how massive some of these lenses were and still are.