drawingdetail: Maerten van Heemskerck, The Piers of the Cross Vault of New St. Peter’s Basilic
drawingdetail:Maerten van Heemskerck, The Piers of the Cross Vault of New St. Peter’s Basilica and the Rest of the Northern Transept of the Old Basilica. Roman Sketchbook I (1532-1536). Pen and ink on paper. Staatliche Museen, Berlin. -- source link