Today we do not doubt the existence of atoms and molecules – we can "see" them using scanning tunnelling and atomic force microscopes. At the beginning of this century, however, the reality of atoms was still a matter for debate. With simple yet brilliant experiments performed between 1907 and 1913, Jean Perrin resolved the issue by studying small particles suspended in solution. Using an ordinary light microscope he studied the behaviour of the smallest objects he could see – spherical colloidal particles of resin just 1 μm across, suspended in water. (A colloidal system contains small particles of one type of material in a continuous matrix of another type. They can be solid in liquid, like paint; liquid in liquid, like milk; and even gas in liquid, such as an aerosol.)