Advances in the field of cell biology have always been closely related to the development of quantitative analytical methods that can be applied to individual cells or cell organelles. Almost from the early stages following the invention of the microscope, the investigator has been keenly interested in obtaining informa- tion on the functionality of single cells and how cells perform under different sets of experimental conditions. Although cells could be viewed in the microscope for a few hundred years, only since the relatively recent application of autoradiography did we come to realize...