Complex sentence: Angel-lights, in architecture, are the upper panes of glass or "lights'" in a curved window frame, next to the springing; probably a corruption of the word angle lights, as they are nearly triangular.
Split into simpler sentences: Angel-lights, in architecture, are the upper panes of glass or "lights'" in a curved window frame, next to the springing. Probably a corruption of the word angle lights, as the structures are nearly triangular.