Weber’s Compass and Aesthesiometers: History of the technical evolution of devices for tactile discrimination

The study of tactile sensitivity necessitates the use of special instruments designed by physiologists and psychologists during the 19^th century and at the turn of the 20^th century. It was the German physiologist Ernst Henrich Weber (1795-1878), who used classic and beam compasses from the late 1820s in order to study tactile sensitivity in humans. He discovered that when the points of a pair of compasses were applied to the skin, the subject’s eyes being closed, they were not felt as distinct separate points unless the inter-compass distance had a certain value, which varied in different regions of the body. The success of Weber’s work, published in 1834, led numerous persons to use compasses in clinical practice and experimental investigations. In the 1840s, the French physiologist and neurologist Charles-Edouard Brown-Sequard (1817-1894) was certainly the first to report the use of Weber's compass in the field of clinical medicine. But technical improvements were rapidly made to the instruments used by Weber. The English physician Edward Henry Sieveking (1816-1904) proposed the use of a new apparatus, called “aesthesiometer”, to determine the degree of anesthesia or paralysis, and its progress towards recovery or the reverse. During the 19th century, many firms, specialized in the manufacture of instruments in the field of medical clinics, offered various types of aesthesiometers in trade catalogs. Since at that time physiologists and psychologists were particularly interested in the study of tactile sensitivity, they invented new more precise instruments. For example, a new type of aesthesiometer characterized by the use of various calibrated monofilaments was manufactured by Max von Frey (1852-1932) in 1894. This instrument was designed to determine the threshold necessary to produce the feeling of touch or pain when the stimulus is applied to the skin with more or less pressure. This paper presents the history of the technical evolution of the compasses and aesthesiometers that were used in most psychology and physiology laboratories at the turn of the century. These instruments have greatly contributed to the emergence of scientific psychology, in particular by allowing the objective study of attention and mental fatigue.