As part of a theory of scientific discovery, to understand the construction of scientific fact, it is necessary to take into account a dimension neglected by historians of science: the transformations of the scientist. Using Horace-Bénédict de Saussure's discovery of the division of infusoria in 1765 as an example, this article shows how these transformations take place and what their nature is. They are reconstructed here through an analysis of his laboratory notebooks, which initially highlight his techniques for visualizing microscopic beings. Gradually, Saussure found himself involved in more powerful processes, in particular envisioning, which consisted in seeing things that one has not learned to know. The elaboration of this process can be traced in the notebooks through the construction of envisioners, i.e. specific categorical-perceptual determinations that enable the scientist to see certain aspects of the hitherto unknown phenomenon and thus to determine it. As a craftsman of his experiments and as an internal worker, the scientist builds new concrete and mental tools and a context of reality that allows him or her to see what others cannot see because they have not built the same tools. This is why, in the face of scientific facts, the envisioners constitutes a form of evidence rather than mere rhetorical persuasion. If the author's task seems to be to convince others of this evidence in order to gain acceptance for the discovery, it remains to be seen whether the actors who will reconstruct it will transform themselves in the same way as the author in order to arrive at a shared scientific fact.