In this paper it is argued that a proper understanding of the justification of perceptual beliefs leaves open the possibility that normal humans, unaided by microscopes, could genuinely know, by direct observation, of the existence of a theoretical entity like an electron. A particular theory of justification called perceptual responsibilism is presented. If successful, this kind of view would undercut one line of argument that has been given (for example, by Bas van Fraassen) in support of scientific anti-realism. Various objections to the idea that electrons can be directly observed are also considered.