In the 1890s Ludwig Mach employed photography for visualizing streamlines in the emerging field of aerodynamic research. Étienne-Jules Marey developed a similar approach at the turn of the century. The two projects can be related to a number of current discussions on the history of scientific photography. The case of Ludwig Mach demonstrates how the collection of numerical data became both the subject and the challenge of a line of research intimately linked to the capacities of photography. At the end (...) of the nineteenth century, the particular potential of scientific photography is very often defined by comparison with the limited power of the human eye. In contrast, the example of streamline photography underlines that the requirements of the research context are critical for successfully employing photography. Marey’s studies point to a tension between his characterization of chronophotography as a method for analyzing the elementary units of processes in nature on the one hand and the necessary summation of single moments in time in his recordings of streamlines on the other. What Marey usually qualified as a cumbersome confusion was here the prerequisite of observation. The ’philosophy in machines’ ultimately limited the success of streamline photography; it aided in debates about qualitative matters, but could hardly provide what most interested scientists and engineers: reliable numbers. (shrink)