The microscope was one of the most profound signifiers of new modes of scientific vision that came to maturity in the nineteenth century. As Graeme Gooday noted in 2008, the microscope became ‘the iconic instrument of laboratory epistemology’, representing access to new knowledge, objectivity and trust in the procedures of scientific experimentation.1 Yet before the microscope attained its position as one of the pre-eminent scientific instruments, which it did only in the last two decades of the nineteenth century, the new ocular horizons it presented to those who put their eye to its lens were unstable and ambiguous. Indeed in one of the key areas of biological research, the study of infectious diseases, the microscope both offered increased hope for the advancement of knowledge and made knowledge unpredictably haphazard. William Henry, the writer of one of the first extended reports on disease for the new British Association for the Advancement of Science in 1834, said in his discussion of infection that while ‘our senses give us no insight into the properties of this subtile [sic] agent’ it is more unfortunate that we do not ‘derive any assistance from the most refined instruments’. It was not that Henry's microscope did not add to his visual capacity, but that it failed to provide either the necessary ‘magnifying power’ or the steady ‘clearness of definition’ needed for thorough investigation.