 The second example stems from early electron microscopy of bacterial cells. Chapman and Hillier (1953) observed invaginations of the plasma membrane in gram-negative bacteria, which they called peripheral bodies. Robertson (1958) renamed them mesosomes and they were implicated by many researchers in a variety of cell functions before being rejected as artifacts in the 1970s (Silva et al. 1976). Analyzing this example, Rasmussen (1993) argues that philosophical criteria for distinguishing real entities from artifacts, such as robustness, are insufficient to explain scientists’ changing judgments about the mesosome. When Chapman and Hillier first made their case for the existence of mesosomes, they had to explain away differences between their micrographs and the observations of light microscopists by arguing that a membrane that only appeared in images with the light microscope was in fact due to the mesosomes being imaged under low resolution. Rasmussen contends they offered this convoluted argument rather than treating the mesosomes as an artifact because they were promoting the new electron microscopes. He further describes how their claims motivated research programs aimed at purifying and biochemically characterizing the mesosome so as to evaluate proposals regarding their function. This initially supported claims to the mesosome’s reality, but other biochemists offered conflicting evidence. In addition, another new technique for preparing specimens for electron microscopy by freezing them, which had its own passionate advocates, generated micrographs that did not show mesosomes. According to Rasmussen, these competing findings, not robustness, determined scientists’ judgments about mesosomes.