How to Grasp an Abstraction: Mathematical Models and Their Vicissitudes between 1830 and 1950. Introduction.- Knowing by Drawing: Geometric Material Models in 19th Century France.- Wilhelm Fiedler and his Models-the Polytechnic Side.- Models from the Nineteenth Century Used for Visualizing Optical Phenomena and Line Geometry.- Modeling Parallel Transport.- The great yogurt project: models and symmetry principles in early particle physics.- Interview...