This project examined the conceptualization of the human senses in observational practices from 1750 to 1830. Starting with a discussion on the task of observation at the time when mechanical registering devices were not available or still rather unreliable, the study concentrated first on the diverging approach taken to the scientific instrument and the human senses in the second half of the eighteenth century. While the instrument was increasingly transformed into a complex, ever error-producing component of scientific observations that had to be investigated and experimented upon, the senses of the observer were mainly understood as something that had to be instructed and trained. The success and trustworthiness of the senses’ work was therefore due to experience and the use of the "right method."
By the beginning of the nineteenth century, it can be claimed that the status of the senses (and the human observer in general) in scientific observations changed. An early attempt at investigating the "error of the sense," a systematic error in the perception of signals, was made by the physicist Johann Friedrich Benzenberg. In contrast to Benzenberg’s approach, which did not draw much attention, Friedrich Wilhelm Bessel’s investigation on differences in measurements between two or more astronomical observers (1819–22) became widely known in the course of the nineteenth century. A detailed case study shows that Bessel originally believed that a well-trained observer contributed only to the accidental errors present in every observation. In this respect the observer’s influence on the data would be the subject of calculus, but when Bessel started some empirical trials he discovered, to his great surprise, the existence of what he called a "constant difference in time" between different astronomers. This was a striking result, but it was not until the middle of the 1830s that the influence of systematic differences between observers became a part of astronomical everyday life.
Three reactions to this work can be noted: (1) a silent one attempting to diminish the phenomena by favouring certain methods of measurement and work organization, (2) one leading to a common routine, consisting of regular measurement of the differences, and (3) one in which the emphasis was put on the development of new technologies. Only in the last case did causes and circumstances of the observed differences play a certain role, while in general, it was sufficient for astronomers to control for the phenomena. The different ways of dealing with Bessel’s discovery finally converged into the idea that the human observer could be understood as a kind of apparatus or “appareil d’observation,” as the French astronomer Le Verrier put it in the mid-1850s, that showed certain characteristics and errors like any other astronomical instrument.
The analogy between the senses of the observer and the observer’s instruments, which appeared in astronomy during the first half of the nineteenth century, was of a completely pragmatic nature. The human observer was likened to an instrument, but sensory organs were not compared to features of instruments nor was their functioning connected to that of instruments. However, studies like Ernst Heinrich Weber’s on the sense of taste or Joseph Plateau’s on the persistence of vision, both begun in the 1820s, prove that at times epistemological framework can change in fundamental ways. The interest in the limitations that are set by the senses in the observation of nature then led to the modeling of the sensory organ or of certain functions of the organ along the line of the instruments used in the investigations. What had been originally studied like an instrument or an apparatus came to be perceived as an instrument or an apparatus. As a result, the notion of an "appareil du sens," originally introduced by the French physiologist François Magendie to describe the functional relation of a number of anatomical entities, came to be understood in a rather literal sense.