This is the old Fresnel light from the interior. It is the largest fresnel light of it's kind. It was built in the 1800's and first unveiled at the same Paris exhibition where the Eiffel Tower was first opened for public use.They have to cover it, because if the sun were to hit it directly the prisms inside the lamp would focus the sunlight in a concentrated beam and melt the interior. Fresnel lights are also used in theatrical lighting, though none on this scale.