those of another observer for the same star. They found a difference of half a second. Later he made similar experiments with Argelander and Struve, with the result of always finding a personal difference.
Bessel sought for the cause of this "personal equation" by varying the conditions. He first made use of the sudden disappearance or reappearance of a star instead of steady motion. The personal difference was much decreased. This seemed to indicate that the trouble lay in comparing the steady progress of the star with the sudden beat of the clock. The next step was to change the beats, with the result that for Bessel the observations were made later with the clock beating half seconds than with one beating seconds, whereas Argelander and Struve showed no particular change. One other point was investigated—namely, the effect of the apparent rate of the star; within wide limits the personal equation was not changed.
About 1838 the personal equation began to receive regular notice in astronomical observations, as appears in the publications of Airy[1] and Gerling of that year.[2]
It was natural to wish for a comparison of the astronomer's record with the real time of transit. At the suggestion of Gauss, an artificial transit was arranged by Gerling, the object observed being a slow pendulum. This is probably the first measurement of a reaction time. In 1854 Prazmowski[3] suggested an apparatus carrying a luminous point for a star and closing an electric circuit at the instant it passed the line; a comparison of the true time with the astronomer's record would give the real amount of his personal equation. From this time onward various forms of apparatus were invented and numerous investigations were carried out. The astronomers found that in such observations sometimes the star was seen to pass the line too soon, sometimes too late, and that the error varied with every variation in the method of observing and in the mental condition of the observer.[4]
Let us turn for a moment to another science. The new physiology, begun by the pupils of Johannes Müller, in which the phenomena of life were to be explained by physical and chemical processes, had undergone a remarkable development. Du Bois-Reymond had taught how to apply the experimental methods and apparatus of physics to the study of physiological processes. Soon after this Helmholtz measured the velocity of nervous transmission (1850), an experiment that Johannes Müller had
