In his annual essay on the philosophy in France for the year 1912, André Lalande (1913, 366-7) made the following observation: M. Perrin, professor of physics at the Sorbonne, has described in Les Atomes, with his usual lucidity and vigor, the recent experiments (in which he has taken so considerable a part) which prove conclusively that the atoms are physical realities and not symbolical conceptions as people have for a long time been fond of calling them. By giving precise and concordant measures for their weights and dimensions, it is proved that bodies actually exist which, though invisible, are analogous at all points to those which we see and touch. An old philosophical question thus receives a positive solution. This brief and matter-of-factly announcement expressed a rather widely shared sentiment on the European continent that Jean Perrin’s experimental work had clinched the issue of the reality of atoms. Indeed, it is now obvious that between roughly 1908 and 1912, there was a massive shift in the scientific community in favour of the atomic hypothesis. As is also obvious that Perrin’s experimental work on the causes of Brownian motion played a major role in this shift. When Perrin received the Nobel Prize for physics in 1926, it was noted in the presentation speech by Professor C W Oseen that he “put a definite end to the long struggle regarding the real existence of molecules”. Peter Achinstein has offered one of the most systematic expositions and reconstructions of Perrin’s argument, aiming a) to show how his own theory of evidence best accounts for the significance of Perrin’s results; and b) how Perrin has offered a local and experimental argument for scientific realism. After some detailed presentation of Perrin’s argument, I will offer my own reconstruction of it and will show why it is superior to Achinstein’s. Finally, I will try to draw some lessons for scientific realism.