The invisible object, in the eighteenth century, is not an evidence. It is the result of textual and semantic learning. Which concrete strategies are used to construct and depict objects out of sight? How do we make them a cognitive reality acceptable to a scientific community? This paper first highlights the conditions for the emergence of a field of microscopic knowledge and its epistemological consequences. Then we consider the microscopic gaze in terms of learning, situated between the act of observation (...) as such and discursive practices. We conclude by studying a concrete case of “negotiation of the invisible” in a correspondence between Carl Linnaeus and John Ellis concerning corpuscles observed in mushroom infusions. (shrink)