 The very idea of a clear-cut theory-observation distinction has received much criticism. First, with the help of sophisticated instruments, such as telescopes and electron microscopes, we are able to observe more and more entities, which had to be considered unobservable at a previous stage of scientific and technical evolution. Electrons and \(\alpha\)-particles which can be observed in a cloud chamber are a case in point (Achinstein 1965). Second, assume observability is understood as excluding the use of instruments. On this understanding, examples drawing on the use of cloud chambers and electron microscopes, which are adduced to criticize the theory-observation distinction, can be dealt with. However, we would then have to conclude that things being perceived with glasses are not observed either, which is counterintuitive (Maxwell 1962). Third, there are concepts applying to or being thought to apply to both macroscopic and submicroscopic particles. A case in point are spatial and temporal relations and the color concepts that play an important role in Newton’s corpuscle theory of light. Hence, there are clear-cut instances of observation concepts that apply to unobservable entities, which does not seem acceptable (cf. Putnam 1962).