Photoelectric cells and mobile apertures on this whole facade are intended to control light levels inside the building, changing as weather conditions change..I don't believe it is working at the moment and the building, now about 30 yearrs old is undergoing a renovation. There is a great bookshop on the ground floor.
The view from the top is wonderful. They are about to position the new spire after the fire of 2019 (which happened the day I came home from my last trip there). From here we walked along the Seine..