THE FRESNEL LENS
The Forty Mile Point Lighthouse tower contains an active 4th Order Fresnel Lens. This size lens is the one most commonly occuring on the Great Lakes.
The brass collar is engraved with the name Henri LePaute, Paris, France and is dated 1872.
According to old 40 Mile Point Lighthouse log books this is probably the third or fourth light installed in the Forty Mile Point tower. While 40 Mile Point lighthouse Park and the Lighthouse are owned and maintained by Presque Isle County through the 40 Mile Point Lighthouse Society, the lens is owned and maintained by the U.S. Coast Guard.
For more information on the Fresnel Lens and how they work, with a list of other excellent references, visit this Wikipedia link