This paper reveals a frogotten formative influence on George Eliot. Most accounts of Eliot's debts to science examine the circle of eminent scientists she and Lewes knew in the 1860s and 1870s and his own late work, Problems of Life and Mind. Here I explore much earlier and less celebrated writing: the microscopical investigations of primitive water creatures that Lewes conducted as an amateur popularizer of science in the mid to late 1850s and the vigorous culture of microscopy to which he introduced George Eliot as early as 1856. After summarizing the technological advances in the microscope that had nurtured this culture and surveying the role of Victorian periodicals in sustaining it, I trace the significance of the discipline, particularly as conveyed in Lewes's neglected article "Only a Pond!," for the texture and structure of Middlemarch. The language of her characters' dialogues teems with details of vocabulary and metaphor first developed by Lewes to map the world of the water-drop onto the equally parasitic relationships of mid-Victorian society. More surprising, Eliot also made her narrator one of the novel's two amateur microscopists, the other being Camden Farebrother, Middlemarch's own amateur natural historian. The pater then explores the different kinds of "advantage" this interest in microscopes secures for Farebrother over Lydgate, the book's representative of professional science, and argues that Farebrother is the novelist's private tribute to Lewes's earlier enthusiams.