Sensational differences: Individuality in observation, experimentation, and representation (France, 1853--1895)

This dissertation examines the problem of individual differences in observation and reaction that arose in nineteenth-century science. This new problem could no longer be explained away by invoking the well-known fallibility of the senses. The inability to perceive and react to the world independently from the individuality of an observer seemed to be parasitical to the very sources of knowledge itself. Individual differences affected elementary perceptions of color, intensity, length and angles and dramatically impacted observations and reactions to moving phenomena. By looking at how measurements, observations, and times of reaction varied amongst individuals, in this dissertation I attempt to track essential changes in what it means to observe and to know. The periodization that I introduce starts during the Second Empire, when the problem became critical in France (1853). It ends with the cinematographic method (1895), by reference to which certain scientists, astronomers and amateurs had sought to solve the problem. I investigate the problem of individual differences in three areas: in the sky, inside bodies, and in things. The first part concerns mostly astronomers and physicists. The second part treats the problem from the perspective of the human sciences. The third and final part brings all of these actors together in an analysis of instruments and imaging techniques that were used to eliminate individual differences. A central intention of my dissertation is to connect my historical account to historiographic questions pertaining to the boundaries between history, philosophy and science. I introduce the concept of frameline as a methodological tool that enables me to relate theories of perception and reaction to current debates in the history and philosophy of science. Using this concept my dissertation explores how the problem of individual variations was related to differences between science and culture, between the exact and the human sciences, instruments and humans, retinas and brains; boundaries often set by invoking the distinction between words and language and felt in bodies considered variously as sensors, reactors, authors and texts.