The text analyzes the related semantics of bacteriology and politics in imperial Germany. The rapid success of bacteriology in the 1880s and 1890s was due not least to the fact that scientific concepts of bacteria as “the smallest but most dangerous enemies of mankind” (R. Koch) resonated with contemporary ideas about political enemies. Bacteriological hygiene was expected to provide answers to social and political problems. At the same time metaphors borrowed from bacteriological terminology were incorporated into the political language of the time. While the “high command of our doctors” (F. J. Cohn) fought diseases, some contemporaries were identified with members of the evil species of “bacillus communis odii.”

Both imperialistic politics and bacteriological science relied on images of inferior and invisible but potent enemies. Both were able to increase their prestige via a mutual interchange of their vocabularies.

The vine louse Phylloxera vastatrix became a “pest” as it was transferred from North America and from France to Germany during the 1870s. Embodying the “invading alien,” it assumed a cultural position that increasingly gained importance in Imperial Germany. In this process, the minute insect, living invisibly underground, was made visible and became constitutive of the scientific-technological object, “pest,” pertaining to a scientific discipline, modern economic entomology. The “pest” phylloxera emerged by being made visible in a way that enabled control measures against it. Thus, visibility functioned as a prerequisite for control measures. I differentiate between social visibility and physical visibility, as well as between social control and physical control of the “introduced pest.” The object phylloxera emerged at the intersection of techniques of social control such as surveillance, techniques of physical control such as disinfection, and representational practices of the sciences such as mathematics and graphics. The space of its visibility was not the vineyard as property of a vintner but the vineyard as national territory, where German (viti-) culture was defended against foreign infiltration and destruction. Many vintners had alternate visions of the grapevine disease, they resented the invasion and destruction of their vineyards by government officers, and thus they did not participate in the social and epistemic constitution of the “pest.” By 1914, the “introduced pest” had not yet become an effective “machinery.” However, the “pest” as an object of scientific knowledge emerged together with economic entomology. The field became organized as a discipline in Germany in 1913, forty years after the phylloxera had first aroused the minds of some worried Wilhelmians, and, together with its nationalistic images, the field of “pest” control became organized towards a redefinition of German society and its perceived dangers.

Microscopical consideration played a crucial role in German physiology in the period of, grosso modo, 1780–1830. Specifically, a conception of material change was established, according to which all life is grounded in the process of the generation of microscopical forms out of an amorphous, primitive generative substance. Embryological development, tissue growth, and the generation of microorganisms were all considered to be the manifestation of this fundamental developmental process. In contrast to the common historiography, I try to understand Theodor Schwann's 1838 discovery of the cell theory in terms of the epistemological categories he applied to the prevailing conceptions of life and living matter. I argue that Schwann was able to discern cells not because of any superior microscopical methods, but rather as part of his wider investigative endeavor to explicate life processes according to specific causal agents. I argue that Schwann was able to demonstrate the existence of cells only when he considered animal tissues in terms of a causal relation between specific material agents and their effect, that is, the developmental history of tissue.

Contrary to the dominant historiography of microscopy, which tends to maintain that there was no microscopical program in the Enlightenment, this paper argues that there was such a program and attempts to illustrate one aspect of its dynamic character. The experiments, observations, and interpretations on rotifers and their management by scholars of that period show that there did exist a precise axis of research that can be followed historically. Indeed, the various controversies these scholars engaged in imply that they performed accurate microscopical experiments and observations and also carefully interpreted their visual data. Furthermore, the kinds of phenomena presented by the rotifer and other microscopic entities, such as their morphology or their revival from desiccation, helped to improve methodology on the use of the microscope. The paper also argues that this sort of inquiry is not fundamentally different from other research in the natural sciences of the eighteenth century.

In this essay I will sketch a few instances of how, and a few forms in which, the “invisible” became an epistemic category in the development of the life sciences from the seventeenth century through the end of the nineteenth century. In contrast to most of the other papers in this issue, I do not so much focus on the visualization of various little entities, and the tools and contexts in which a visual representation of these things was realized. I will be more concerned with the basic problem of introducing entities or structures that cannot be seen, as elements of an explanatory strategy. I will try to review the ways in which the invisibility of such entities moved from the unproblematic status of just being too small to be accessible to the naked or even the armed eye, to the problematic status of being invisible in principle and yet being indispensable within a given explanatory framework. The epistemological concern of the paper is thus to sketch the historical process of how the “unseen” became a problem in the modern life sciences. The coming into being of the invisible as a space full of paradoxes is itself the product of a historical development that still awaits proper reconstruction.

This paper is concerned with the diversity of microscopic research in nineteenth-century life sciences. It examines how two researchers, Ernst Wilhelm Brücke and Heinrich Müller, investigated the structure and function of the retina. They did so in significantly different ways, thereby developing quite different accounts of this organ and its role in the process of vision. Both investigators were carrying out microscopic investigations, both were particularly concerned with interpreting their findings in terms of physiological function, and both employed the physical sciences in their microscopic research. Their approaches differed, however, with respect to the manner of handling and preparing the tissues, as well as with respect to the conceptual tools they applied to their findings.

The cases indicate that the common tendency to associate microscopic research mainly with morphological studies of organic material is not appropriate. To understand nineteenth-century microscopy and its place in the sciences of life, close attention should be paid to the manner in which microscopic investigations were performed. It is only then that the flexibility and versatility of microscopic research comes into view.