 Van Fraassen argues for this position in part by describing Millikan’s famous experiment measuring the charge of the electron. Scientific realists take this experiment to be making a discovery about the nature of the unobservable entities known as electrons. Van Fraassen, in contrast, presents the experiment as “filling in a value for a quantity which, in the construction of the theory, was so far left open” (1980, 77). In doing the experiment, Millikan was discovering a regularity in the observable part of the world, and was providing a value for a quantity in atomic theory. Millikan need not be understood as discovering something about the nature of unobservable objects in the world. Van Fraassen says that in a case like Millikan’s,