This book provides novel insights into the practices of representing invisible objects in nineteenth-century and twentieth-century laboratory sciences. It tackles questions such as: How did scientific practitioners make sense of mathematical representations of theoretical entities, and did their understanding depend on transformations of mathematical sign systems into diagrams, graphs or other iconic modes of representation? Are modes of representation conceptually essential or merely decorative features of scientific discourse? Why did experimental scientists implement theoretically loaded sign systems, such as chemical formulas, in their practical activities, and what were the functions of such sign systems in experimental practice? The essays contained in this volume carefully follow the way scientists constructed, juxtaposed and transformed representations of invisible objects of inquiry, and explore the pragmatic use of representations as tools in scientific and industrial practices. Historians and philosophers of science, but also experimental scientists interested in the epistemological, semiotic and historical issues of their discipline, will find theoretical propositions about representations as well as a multifaceted portrayal of scientists' constructions and applications of representations - be they the structural formula of a dye, the three-dimensional model of a protein, a table conveying relationships between chemical elements, a diagram depicting the functional relationships of the genetic apparatus, or a lengthy text dealing with the molecular level of objects.