Phages, antibodies and de-monstration

Using examples from the field of molecular genetics and immunology, this paper examines the argumentative strategy underlying the use of electron micrographs as decisive evidence for previously uncertain or disputed claims. Scientists often resort to visual imagery in order to demonstrate the factual status of their claims and thus to compel assent from their peers, therefore bypassing other forms of argumentation such as propositional reasoning. The particular form of demonstration discussed in this article resorts to the use of photography rather than drawings and diagrams. While the mechanical objectivity of micrographs certainly adds to their evidential strength, the pictures we examine derive part of their power from their arrangement in a sequence that mimics experimental operations. The visual argument tracks the textual report of the steps in a biochemical or molecular genetic experiment, with which it becomes intimately associated. We conclude that much of the evidential strength conveyed by the articles we analyze is to be sought less in the extrinsic attributes of the instrumentation they mobilized than in the specific material and argumentative practices enacted by those particular uses of the instrument.