“A Vitious Way of Observing”: Kinnebrook and the Prehistory of the Personal Equation

David Kinnebrook is remembered as the assistant to the Astronomer Royal dismissed in 1796 for marking stellar transits too slowly. Kinnebrook’s firing is commonly listed as the origin point for the personal equation and empirical psychology. Historians have viewed Kinnebrook as a slightly misused, although mute, party in the affair, drawing their accounts from Nevil Maskelyne’s published account of the dismissal. Kinnebrook’s letters, which resurfaced in 1985, detail a monthslong dispute with Maskelyne concerning observational disagreements. The letters provide an account of observational practice and individual differences in marking transits; they also detail Maskelyne’s failure to recognize an epistemic problem. Kinnebrook’s letters show that the problem of individual differences was known to Maskelyne as well as several of his contemporaries. Evidence from the letters shows that Maskelyne passed off six weeks of observations collected by his disgraced assistant as his own. The letters also state that Maskelyne had substantial personal observational differences with previous assistants. While Bessel later conjectured that Kinnebrook was under pressure to speed up his observations, Kinnebrook’s letters confirm that conjecture and reveal that Kinnebrook had a deeper comprehension of the epistemological problems of observation than has been recognized. Kinnebrook’s recognition of personal differences in marking stellar transits is a significant moment in the prehistory of the personal equation.